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:
- I took a good-size snort out of that big bottle [of furniture polish] in the middle....Have you none of the clear McCoy handy around the house?
The term is the subject of many folk etymologies, discussed at the Wikipedia entry.
the real McKoy
- (idiomatic) The genuine thing, neither a substitute nor an imitation.
- ^ Scottish National Dictionary
- ^ 2007 OED
- ^ Susie Dent of the Oxford University Press, on February 8, 2008 broadcast of Countdown.
- ^ The Mavens’ Word of the Day: real McCoy cites Dictionary of Americanisms, which gives the citation for this quote as Davenport, Butte Beneath X-Ray.