Wordorigins.org

This term meaning the genuine article derives from a brand of whisky. The phrase the real MacKay, referring to a brand of whisky of that name, appears in 1856. It was officially adopted as an advertising slogan by G.Mackay and Co. of Edinburgh in 1870. When it crossed the Atlantic the spelling shifted to become McCoy. From the poem Deil’s Hallowe’en, published in 1856:

A drappie o’ the real McKay.