Definition
The genuine thing, neither a substitute nor an imitation.
Origins
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.
Also Said As
How People Actually Use It
Real deal. Oilers on early trains had to be oiled to stop friction on the drive wheels. Oilers always failed... except the McCoy brand and they became known as the best and 'the real McCoy'
"These appear to be the real McCoy."
Source: Urban Dictionary