The expression ‘the Real McCoy’ – meaning the genuine article – was first used in Scotland in the form of ‘real McKay’ where it was applied to first class whiskey. When the whiskey was exported to the United States, the expression accompanied it. At some time, the pronounciation was corrupted to McCoy.
The name was also used by a boxer whose real name was Norman Selby (1873 – 1940). Selby began his boxing career in 1891, employing the pseudonym Charles ‘Kid’ McCoy in an attempt to capitalise on the popularity that Irish boxers enjoyed in the United States at that time. In March 1896, he won the world welterweight championship by beating Tommy Ryan, a genuine Irishman.
During the course of a successful career, moving up the weight classes (eventually to heavyweight) and at the height of his success, a middleweight boxer by the name of Al McCoy appeared on the circuit. From then on, Selby was billed as the ‘Real McCoy’ to distinguish him from the other.
Source: Eileen Hellicar (1982): The Real McCoy. Readers Digest Association. ISBN 0-949819-55-7