as in the real McCoy, 1881, said to be from Scottish the real Mackay (1883), of uncertain origin, though there are many candidates, including whiskey distilled by A. and M. Mackay of Glasgow (the phrase the real McCoy became popular during Prohibition to describe liquor); Charles S. "Kid" McCoy (1872-1940), former welterweight boxing champ; and a claimant for chief of the northern branch of the clan Mackay.
"By jingo! yes; so it will be. It's the 'real McCoy,' as Jim Hicks says. Nobody but a devil can find us there." [James S. Bond, "The Rise and Fall of the Union Club," Yorkville, Canada, 1881]
The best of its kind, the real thing: “That homemade pizza was the real McCoy.” The source of this expression is the story of a famous prizefighter named McCoy. He had so many imitators that no one was sure which was the real one.
Genuine; legitimate; kosher: like every other McCoy biz (1930+)