Stories We Like: Novels For Language Lovers
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]
Genuine; legitimate; kosher: like every other McCoy biz (1930+)