How “MacKay” became “McCoy” is unclear; first attested with this spelling in Canada in 1881. In James S. Bond's The Rise and Fall of the "Union club": or, Boy life in Canada, a character utters, "By jingo! yes; so it will be It's the 'real McCoy,' as Jim Hicks says. Nobody but a devil can find us there."
The term is the subject of many folk etymologies, discussed at the Wikipedia entry.
- ^ Scottish National Dictionary
- ^ 2007 OED
- ^ Bond, James S. The rise and fall of the "Union club": or, Boy life in Canada. Yorkville, Ontario. p. 1