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() Corruption of Scots the real MacKay, this latter attested in 1856 as “A drappie o’ the real MacKay” (A drop of the real MacKay).Scottish National Dictionary2007 OEDSusie Dent of the Oxford University Press, on February 8, 2008 broadcast of Countdown. 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 in the middle....Have you none of the clear McCoy handy around the house? cites Dictionary of Americanisms, which gives the citation for this quote as Davenport, Butte Beneath X-Ray. The term is the subject of many folk etymologies, discussed at the Wikipedia entry.