For more than 300 years "rule of thumb" has meant what most people think it means: any rough-and-ready method of estimating. It's believed to have originated with woodworkers, who made measurements with their thumbs. --This answer if from The Straight Dope, whose research I trust completely.

Recall that an inch was once defined as the length of the thumb (the king's would be the standard) from the first joint to the tip. It's still a pretty good approximation. And of course the "foot" was the lenth of the king's foot. Which is why the thing you would measure it with was called a ruler.

A couple of hundred years ago a man could use physical punishment to get his wife to behave. But if you disabled her, that was breaking the law. A supple branch of a tree would deliver a painful snap without risking broken bones. But bigger branches did break bones so you could not use a branch thicker than your thumb. That is simple enough so all men could understand. No beatings with a supple branch that is thicker than a thumb. It is ironic to hear a women at a equal rights gathering to use this phrase. Some saying might have several origins that one might be unaware of.

A rule of thumb is basically a method or principle that is used as a rough and ready practical rule, and doesn't have a precise, scientific basis. A lot of people believe that the saying came from an Old English law that allowed a man to beat his wife with a stick no wider than his thumb. There are other theories on how the saying came to be, including it's use as a physical measurement and it's use in testing temperature.

rule for an emergency

1. common practice: a way of proceeding based on experience or sound judgment 2. generally reliable method: any practical, though not entirely accurate, method that can be relied on for an acceptable result [Said to derive from the workman’s practice of using the thumb as a rough measure]

Meaning: A means of estimation made according to a rough and ready practical rule, not based on science or exact measurement. Origin This has been said to derive from the belief that English law allowed a man to beat his wife with a stick so long as it is was no thicker than his thumb. In 1782 Judge Sir Francis Buller is reported as having made this legal ruling. The following year James Gillray published a satirical cartoon attacking Buller and caricaturing him as 'Judge Thumb'. The cartoon shows a man beating a fleeing woman and Buller carrying two bundles of sticks. The caption reads "thumbsticks - for family correction: warranted lawful!" It seems that Buller was hard done by. He was notoriously harsh in his punishments, but there's no evidence that he ever made the ruling that he is infamous for. Edward Foss, in his authoritative work The The Judges of England, 1870, wrote that, despite a searching investigation, "no substantial evidence has been found that he ever expressed so ungallant an opinion". It's certainly the case that, although British common law once held that it was legal for a man to chastise his wife in moderation (whatever that meant), the 'rule of thumb' has never been the law in England. Despite the phrase being in common use since the 17th century and appearing many thousands of times in print, there are no printed records that associate it with domestic violence until the 1970s. The false stories that assumed the wife-beating law to be true may have been influenced by Gillray's cartoon. Even if people mistakenly believed that law to exist, there's no reason to connect the legal meaning with the phrase - which has been in circulation since at least 1692, when it appeared in print, in Sir William Hope's training manual for aspiring swordsmen, The Compleat Fencing-master, 1692: "What he doth, he doth by rule of Thumb, and not by Art." The origin remains unknown, although it is likely that it refers to one of the numerous ways that thumbs have been used to estimate things - judging the alignment or distance of an object by holding the thumb in one's eye-line, the temperature of brews of beer, measurement of an inch from the joint to the nail to the tip, or across the thumb, etc. The phrase joins the whole nine yards as one that probably derives from some form of measurement but which is unlikely ever to be definitively pinned down. http://www.phrases.org.uk/meanings/rule-of-thumb.html RULE OF THUMB - "There are two good choices here. Brewmasters of old often tested the temperature of a batch of beer by dipping a thumb in the brew, their long experience telling them how well the beer was brewing. One theory has it that our expression for a rough, guesswork estimate derives from this practice. More likely it stems from the ancient use of the last joint of the thumb as a measuring device for roughly one inch." From the "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997). A second reference also lists these two theories and adds a couple of details. The lower part of the thumb is roughly one inch long in the average adult male. And what the brewmaster was checking was the temperature of the beer, a practice that was "neither so accurate nor so hygienic as a thermometer check." From the "Morris Dictionary of Word and Phrase Origins" by William and Mary Morris (HarperCollins, New York, 1977, 1988). http://www.phrases.org.uk/bulletin_board/40/messages/592.html Hereby hangs a tale (possibly even a tale and a half). The phrase "rule of thumb" is notable today, not for its real origin, but for a modern myth of its origin. Supposedly, under English common law in the 17th century, the original "rule of thumb" allowed a man to beat his wife with a switch on the condition that the switch be no thicker than his thumb. Thus, it is said, the phrase is inherently oppressive and offensive and should never be used. Now, while I believe this story to be untrue, the general question of "hidden" offensiveness in idioms is a legitimate one. Many of our words and phrases are painfully potent reminders of attitudes and practices of the past that we find reprehensible today. On the other hand, some of our idioms have traveled so far from their nefarious origins as to have earned a reprieve. Thus, a phrase such as "indian giver" may rightly still be considered offensive today, while "gyp" has largely lost its overtones of the once-common prejudice against gypsies. There is no simple rule for deciding whether a phrase has lost its sting -- only our reasonable sensitivity and good will tempered by a healthy resistance to the shrieking paranoia so commonly found in discussions of language these days. In this case, the issue is moot, because the "sexist origin" of this phrase is almost certainly pure invention. "Rule of thumb" probably came from the use of the thumb as a convenient measuring tool, the distance to the first knuckle usually being about one inch. Even "The Bias-Free Word Finder," the bible of the Politically Correct Language Guardians among us, considers the wife-beating theory implausible and notes that it first surfaced in a 1986 letter to the editor in "Ms." magazine. So I guess the first "rule of thumb" in these cases is "Check your sources, lest they be hokum." http://www.word-detective.com/back-n.html#rulothumb This sounds like the invention of somebody desperately trying to make sense of a traditional phrase — what linguists call folk etymology. And it’s quite certainly untrue. But there’s a lot more to it than just fevered imagination. The expression rule of thumb has been recorded since 1692 and probably wasn’t new then. It meant then what it means now — some method or procedure that comes from practice or experience, without any formal basis. Some have tried to link it with brewing; in the days before thermometers, brewers were said to have gauged the temperature of the fermenting liquor with the thumb (just as mothers for generations have tested the temperature of the baby’s bath water with their elbows). This seems unlikely, as the thumb is not that sensitive and the range of temperatures for fermentation between too cool and too warm is quite small. It is much more likely that it comes from the ancient use of bits of the body to make measurements. There were once many of these: the unit of the foot comes from pacing out dimensions; the distance from the tip of the nose to the outstretched fingers is about one yard; horse heights are still measured in hands (the width of the palm and closed thumb, now fixed at four inches); and so on. There was an old tailors’ axiom that “twice around the thumb is once around the wrist”, which turns up in Gulliver’s Travels. It’s most likely that the saying comes from the length of the first joint of the thumb, which is about an inch (I remember once seeing a carpenter actually make a rough measurement this way). So the phrase rule of thumb uses the word rule in the sense of ruler, not regulation, and directly refers to this method of measurement. So where does beating your wife come in? Sharon Fenick wrote an article about its origins in the newsgroup alt.folklore.urban in 1996. She found that for more than two centuries there have been references in legal works to the idea that a man may legally beat his wife, provided that he used a stick no thicker than his thumb; but the references were always to what some people believed, not to established legal principle. The British common law had long held that it was legal for a man to chastise his wife in moderation, as one might a servant or child, but Sir William Blackstone wrote in his Commentaries on the Laws of England in 1765 that this principle was by then in decline. So far as I can discover nothing was ever laid down about how such discipline should be applied. Ms Fenick traced the idea back to a pronouncement that was supposed to have been made in 1782 by a British judge, Sir Francis Buller; this led to a fiercely satirical cartoon by James Gillray that was published on 27 November that year, in which Buller was caricatured as Judge Thumb. (Buller was a brilliant lawyer, the youngest man ever to be appointed a judge in Britain, at 32, but he was widely considered hasty and prejudiced in his opinions.) It might be that he never made the statement that rendered him so notorious. Edward Foss, in his Biographical Dictionary of the Judges of England of 1864 says that to Buller “is attributed the obnoxious and ungentlemanly dictum that a husband may beat his wife, so that the stick with which he administers the castigation is not thicker than his thumb”, but says he can’t find any evidence Buller said it. But the Dictionary of National Biography and other standard works say firmly he did, as did contemporary biographies. However, it was only in 1976, so far as I can discover, that the traditional phrase rule of thumb became directly associated with this spurious legal maxim, through a bit of wordplay in a report that was misunderstood by readers. It is extraordinary that we can so accurately pinpoint the moment at which this folk belief came into being. And how astonishing, too, that it should have survived more than two centuries to become part of the folklore of modern times. http://www.worldwidewords.org/qa/qa-rul1.htm The earliest citation comes from Sir William Hope’s The Compleat Fencing-Master, second edition, 1692, page 157: "What he doth, he doth by rule of thumb, and not by art." The term is thought to originate with wood workers who used the length of their thumbs rather than rulers for measuring things. It is often claimed that the term originally referred to a law that limited the maximum thickness of a stick with which it was permissible for a man to beat his wife, but this has been partially discredited. Famously, a British judge, Sir Francis Buller, was alleged to have stated that a man may legally beat his wife, provided that he used a stick no thicker than his thumb. However, it is questionable whether Buller ever made such a pronouncement and there is even less evidence that he phrased it as a "rule of thumb"; the rumoured statement was so unpopular that it caused him to be lambasted as "Judge Thumb" in a satirical James Gillray cartoon. The "rule of thumb" was referenced in at least four legal cases from 1782 to 1897, and in each of the known cases it was referred to only to state its invalidity, with one judge calling it "...a barbarous custom which modern authorities condemn." "It's certainly the case that, although British common law once held that it was legal for a man to chastise his wife in moderation (whatever that meant), the 'rule of thumb' has never been the law in England." In the modern period, this non-law gained popularity after feminist Del Martin wrote in 1976: Our law, based upon the old English common-law doctrines, explicitly permitted wife-beating for correctional purposes. However ... the common-law doctrine had been modified to allow the husband 'the right to whip his wife, provided that he used a switch no bigger than his thumb'--a rule of thumb, so to speak. http://en.wikipedia.org/wiki/Rule_of_thumb 1. common practice: a way of proceeding based on experience or sound judgment 2. generally reliable method: any practical, though not entirely accurate, method that can be relied on for an acceptable result Probable origin: the practice of using the thumb as a rough measure since ancient times. http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=1861706471

This British slang phrase has been said to derive from the belief that English law allowed a man to beat his wife with a stick so long as it is was no thicker than his thumb.

Unknown, but the most likely candidate comes from the woodworkers of the middle ages that used the tip of their thumb to measure out an inch. I want to point out that it does NOT come from any law about beating one's wife. Let the hoax die already. http://www.debunker.com/texts/ruleofthumb.html

"The 'rule of thumb'turns out to be an excellent example of what might be called a feminist fiction....According to Canadian folklorist Philip Hiscock, 'The real explanation of 'rule of thumb'is that it derives from woodworkers....who knew their trade so well they rarely or never fell back on the use of such things as rulers. Instead, they would measure things by, for example, the length of their thumbs." (Sommers, 1994)It has nothing to do with wife-beating. Sommers, C. (1994). Who Stole Feminism. New York, New York: Simon & Schuster.

Another one I heard was that an inch was defined as the width of the kins thhumb knuckle.