Word Origin & History
McCoy
as in the real McCoy, 1922, from Scot. 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.