It comes from the 1890s/1900s and is about a light heavyweight boxer called Kid McCoy. (Charles 'Kid' McCoy)He was often immitated by other fighters and to prove a point he knocked out a very tough opponent in the first round of a major fight and went over to the press table and shouted 'Now you've seen the real McCoy'

My wife is a direct descendant of the Hatfield clan, involved in the Hatfield-McCoy feud. She insists it comes from that but can't give me any more detail than that. Sorry it's not much of an answer, maybe someone can expound on that or maybe it's completely wrong... :)

I don't think that we can be really sure... 1) "Joseph McCoy (December 21, 1837 – October 19, 1915) was a 19th century cattle baron. Born in Sangamon county, Illinois, he is often cited as the inspiration for the phrase "The Real McCoy" because of his reputation and reliability and because he referred to himself by that phrase (others say the real honor goes to Elijah McCoy and his oft-imitated lubrication system, the boxer Kid McCoy, or other candidates). Joseph McCoy made good on his pledge to Texas ranchers that if they would drive their Longhorn cattle from Texas to Kansas that he would have them shipped by rail to other markets and that the ranchers would receive a good price for their stock." "McCoy advertised extensively throughout Texas to encourage cattle owners to drive their cattle to market in Abilene. By 1870 thousands of Texas longhorn cattle were being driven over the Chisholm Trail to the shipping center at Abilene. By 1871 as many as 5,000 cowboys were being paid off during a single day, and Abilene became known as a rough town in the Old West. Due to their long legs and hard hoofs, Longhorns were ideal trail cattle, even gaining weight on their way to market. One story says that McCoy bragged before leaving Chicago that he would bring 200,000 head in 10 years and actually brought two million head in 4 years, leading to the phrase "It's the Real McCoy"" Source and further information: http://en.wikipedia.org/wiki/Joseph_McCoy 2) "William Frederick McCoy (died December 30, 1948), also known as "Bill" McCoy, was an American sea captain and rum runner smuggler during the Prohibition in the United States." "During Prohibition (1920-33), the McCoy brothers fell on hard times. Their excusion and freight business could not compete with the new highways and buses being built up and down the coast and across Florida. Needing money, the two brothers made a decision to go into rum-running. They sold the assets of their business, traveled to Gloucester, Massachusetts, and bought the schooner Henry L. Marshall. After a few successful trips smuggling liquor off the coast of the United States, Bill McCoy had enough money to buy the schooner Arethusa. Placing the ship under British registry, in order to avoid being subjected to U.S. law, Bill had to change the name from Arethusa to Tomoka (after the name of the River that runs through his hometown of Holly Hill). McCoy made a number of successful trips aboard Tomoka and along with the Henry L. Marshall, and upwards of five other vessels, hauling mostly Rye, Irish and Canadian whisky as well as other fine liquors and wines, McCoy was becoming a household name, and an enemy of the U.S. Government and organized crime. McCoy's legend grew as his quality liquor and fair-dealing perpetuated the phrase, "it's the real McCoy."" Source and further information: http://en.wikipedia.org/wiki/William_S._McCoy 3) " A number of different theories attempt to explain the origin of the phrase. Two of the most popular theories revolve around alcohol. One suggests that the original saying was "the real MacKay," an advertising slogan that appeared in 1856 for a Scottish whiskey. Eventually, this saying supposedly evolved to "the real McCoy" in the United States. The second alcohol-related theory poses that Bill McCoy was a bootlegger in the U.S. during Prohibition. Hence real booze became known as "the real McCoy." Others claim the saying refers to Elijah McCoy, an engineer born in 1844. He invented the self-regulating lubricator, which revolutionized the industrial machine and railroad industry. Because his product was so reliable and many inferior products were produced, buyers of the lubricator would ask, "Is this the real McCoy?" Yet another explanation centers on welterweight boxer Norman Selby, also known as "Kid McCoy." Reportedly, a blow from the boxer left one victim proclaiming, "It's the real McCoy!" Although these are the most oft-repeated, they are by no means the only possibilities." Source and further information: http://ask.yahoo.com/20010601.html 4) ""There are at least half a dozen theories about which of the myriad McCoys of America at the end of the nineteenth century is the genuinely real McCoy. Was it, as Alistair Cooke argued, a famous cattle baron? Or was it perhaps Elijah McCoy, who invented a machine to lubricate the moving parts of a railway locomotive? The broad consensus seems to be that it was Kid McCoy, the former welterweight boxing champion of the 1890s. He had so many imitators, taking his name in boxing booths in small towns throughout the country, that it seems he had eventually to bill himself as Kid "The Real" McCoy, and the phrase stuck. The Oxford English Dictionary records this from a letter written by the author Robert Louis Stevenson in 1883: "He's the real Mackay". It's not only in a different spelling, but a decade before Kid McCoy became famous, and almost certainly refers to the famous Scottish firm of whisky makers." and... "The real McCoy is the genuine article. The saying is said to have arisen in the late 1920s. At that time there was a well known boxer called Kid McCoy; a drunk picked a fight with him without realising who he actually was. When told the situation he was still unbelieving. In the end Kid McCoy knocked him out; on waking up the drunk is alleged to have said; " you're right, it's the real McCoy!" Like many of these sayings there is another suggested origin. This goes back to the 1880s and uses the name McKay, which was the name of a brand of whisky and was advertised as "The real McKay". A third alternative comes from the days of prohibition in the USA. Billy McCoy was an infamous smuggler of hard liquor from Canada into the eastern US seaboard. Since the articles were genuine and not home-brewed the they were "real McCoys" The phrase is recorded in an Irish ballad of the 1880s and was in use in Australia in 1903. The real McKay is pretty certainly the correct version." " Source and further information: http://www.phrases.org.uk/bulletin_board/13/messages/1188.html