Green’s Dictionary of Slang

1. first-rate whisky or beer; often as clear McCoy.

1908		W.G. Davenport Butte and Montana beneath the X-Ray 20: I took a good-sized snort out of that big bottle of furniture polish in the middle. [...] Have you none of the clear McCoy handy around the house? [DA].
1931–4		D. Clemmer Prison Community (1940) 334/1: McCoy, n. Genuine liquor.
1949		Monteleone Criminal Sl. (rev. edn).

2. (US) money.

1935

N. Algren Somebody in Boots 253: We want some place where ain’t no light lit, where ain’t no dog, whare ain’t nobody sleepin’. And where is lots of the ol’ McCoy.

3. (US drugs) medicinal drugs; pure narcotics.

1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 106/1: McCOY. Medicinal drugs; hence chemically pure drugs as contrasted to bootleg drugs.
1959		J.E. Schmidt Narcotics Lingo and Lore.