The real McCoy

"The real McCoy" is an idiom used throughout much of the English-speaking world to mean "the real thing" or "the genuine article" e.g., "he's the real McCoy". Its origins, though generally thought to be nineteenth or early twentieth century, are somewhat obscure.

In Scotland the reference is always the real MacKay (with the ay pronounced as i).. In Ireland this changed to McCoy. The Irish MacKays, McCoys and Magees originated in Scotland, crossing to the Ulster Plantations in the 17th century.

Origins

Michael Quinion of the "World Wide Words" website summarises the half dozen or so theories on the origin of this phrase:
* A boxer, Norman Selby, known as Kid McCoy, American welterweight champion from 1898–1900. There are apocryphal tales to the effect that he had many imitators and had to adopt the term to distinguish himself. Others say that during one match, he pretended to be dazed and weak after being hit in order to trick his opponent into attacking him. But then he came back and surprised his opponent with an attack, and the announcer said "which is the real McCoy?"
* "The Real MacKay," a phrase that appeared first in 1856 as "A drappie [drop] o’ the real MacKay," by the Scottish National Dictionary; the same work says that the phrase was later adopted as a slogan to promote G Mackay & Co Ltd's whisky. The Oxford English Dictionary quotes Robert Louis Stevenson from 1883 in a letter saying "He's the real Mackay."
* The McCoy family of an infamous family feud between the Hatfields and the McCoys on the West Virginia-Kentucky border in the United States in the late nineteenth century.
* A famous American cattle baron by the name of Joseph McCoy (Alistair Cooke's theory).
* During the U.S. Prohibition era, it was common for rum-runner captains to add water to bottles to stretch their profits, or to re-label it as better goods. One American rum-runner captain and boat builder, William S. McCoy, became famous for never watering his booze, and selling only real top-quality products. Because of this, some accounts place McCoy as the source of the term "the Real McCoy."
* A reference to pure heroin imported from Macau.
* Elijah McCoy (1844–1929), Black Canadian inventor of a lubrication system for steam engines. Supposedly, after failed attempts by competitors to make counterfeits of his lubricant, the phrase "real McCoy" was used to refer to his authentic product. Engine lubricators with his name on them were not manufactured until the 1920's.

Quinion notes that many authorities favor the Kid McCoy story, but he personally finds the MacKay story more convincing because of the concrete evidence which generally predates the references supporting other stories.

The term "the real McCoy" has become a cliché and frequently appears in headlines, titles of works, and other contexts.

In his 1991 book "D.B. Cooper: The Real McCoy", former FBI agent Russell Calame proposed that a man named Richard McCoy, Jr. was actually the vanished airplane hijacker D.B. Cooper.

References

* [http://www.worldwidewords.org/qa/qa-mcc1.htm World Wide Words] summary of theories.