Green’s Dictionary of Slang

(US) genuine.

1928		New Yorker 15 Dec. 55: Everything looks McCoy, see? [HDAS].
1931		Irwin Amer. Tramp and Und. Sl. 127: McCoy.—Neat; good-looking; unusually excellent or genuine.
1942		R. Casey Torpedo Junction 120: General quarters sounded with the beating of gongs that certified it to be a McCoy alarm.
1962		Ragen & Finston World’s Toughest Prison 808: McCoy – Neat; good-looking; unusually excellent or genuine.