real McCoy

Meanings

1. The genuine thing, neither a substitute nor an imitation.

Etymology

Corruption of Scots the real MacKay, this latter attested in 1856 as “A drappie o’ the real MacKay” (A drop of the real MacKay). How “MacKay” became “McCoy” is unclear; first attested in the form “McCoy” in the US in 1908. The term is the subject of many folk etymologies, discussed at the Wikipedia entry.