Where does the expression 'more...than you can shake a stick at' come from?

Its first recorded appearance is found in The Lancaster (Pennsylvania) Journal in 1818: "We have in Lancaster as many Taverns as you can shake a stick at." The sense then, as now, was "a lot" or "too many to count."
Shaking a stick at someone has long been considered, for good reason, a threatening gesture. There is a possibility that "more than you can shake a stick at" first arose in the context of warfare or smaller-scale hostilities, perhaps describing an overwhelmingly superior opposing force (e.g., "More Redcoats than you could shake a stick at.").

Another possibility is the stick in question was shaken in the process of counting great numbers of something, perhaps used as a pointer while doing a head count of a herd of sheep or cattle. Thus, "more than you can shake a stick at" would simply mean, figuratively, "you could wave your counting stick until your arm falls off, and you still wouldn't reach the end."

http://www.phrases.org.uk/bulletin_board/5/messages/474.html

Merriam-Webster staff opined that the "count" interpretation
"works as well for 'as many as you can shake a stick at' [...] if
you take it to mean that there is no limit to how many of the
objects in question one could shake one's stick at. [...] We would
consider 'A can't shake a stick at B' a different expression
entirely, with a meaning similar to 'A can't hold a candle to
B' [...]."

In their 1897 work A Dictionary of Slang, Jargon & Cant,
Albert Barrere and Charles Leland suggested that Dutch immigrants
originated the expression using the Dutch word schok = "to shake
or hit."

http://alt-usage-english.org/excerpts/fxmoreth.html

http://idioms.thefreedictionary.com/more+than+you+can+shake+a+stick+at

The question, of course, is why one would be counting or measuring a crowd of something by shaking a stick at it. Shaking a stick at someone has long been considered, for good reason, a threatening gesture. There is a possibility that "more than you can shake a stick at" first arose in the context of warfare or smaller-scale hostilities, perhaps describing an overwhelmingly superior opposing force (e.g., "More Redcoats than you could shake a stick at.").

Another possibility, and one that I find more likely, is the stick in question was shaken in the process of counting great numbers of something, perhaps used as a pointer while doing a head count of a herd of sheep or cattle. Thus, "more than you can shake a stick at" would simply mean, figuratively, "you could wave your counting stick until your arm falls off, and you still wouldn't reach the end."

http://www.word-detective.com/100699.html

From http://www.worldwidewords.org/qa/qa-sha2.htm

Its recorded history began — at least, so far as the Oxford English Dictionary knows — in the issue of the Lancaster Journal of Pennsylvania dated 5 August 1818: “We have in Lancaster as many Taverns as you can shake a stick at”. Another early example is from Davy Crockett’s Tour to the North and Down East of 1835: “This was a temperance house, and there was nothing to treat a friend that was worth shaking a stick at”. A little later, in A Book of Vagaries by James K Paulding of 1868, this appears: “The roistering barbecue fellow swore he was equal to any man you could shake a stick at”.

The modern use of the phrase always exists as part of the extended and fixed phrase “more ... than you can shake a stick at”, meaning an abundance, plenty. The phrase without the “more than” element is rather older, but not by much.

Shaking a stick at somebody, of course, is a threatening gesture, or at least one of defiance. So to say that you have shaken a stick at somebody is to suggest that person is an opponent, perhaps a worthy one. The sense in the second and third quotations above seem to fit this idea: “nothing worth shaking a stick at” means nothing of value; “equal to any man you could shake a stick at” means that the speaker is equal to any man of consequence.

Where it comes from can only be conjecture. One possibility that has been put forward is that it derives from the counting of farm animals, which one might do by pointing one’s stick at each in turn. So having more than one can shake one’s stick at, or tally, would imply a great number. This doesn’t fit the early examples, though, which don’t have any idea of counting about them. Another idea is that it comes from battle, in which one might shake a stick at one’s vanquished enemy. This could possibly have led into the early usages.

Following publication of this piece in the World Wide Words newsletter, Suzan Hendren and Sherwin Cogan suggested that it might have come from the Native American practice of counting coup, in which merit was gained by touching a vanquished enemy in battle. In that case, “too many to shake a stick at” might indicate a surplus of fallen enemies, and “not worth shaking a stick at” would equate a person with “an enemy who is so cowardly or worthless that there is no merit to be gained from counting coup on him”, as Sherwin Cogan put it. An intriguing idea, but there’s no evidence that I know of.

Let me summarise: nobody knows for sure.

I don't know but it really should be retired.

I'd have to say that it most likely is from an old Dutch saying/Lancaster County. My Mom used to say it a lot...another one of her's was "you can catch more flies with honey than you can with vinegar." I'm guessing that this, too was a "Dutchified saying." She was quite German.