Anciently, a fool. Fools, in great families, wore a cap with bells, on the top of which was a piece of red cloth, in the shape of a cock’s comb. At present, coxcomb signifies a fop, or vain self-conceited fellow.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Cow’s Thumb * CrabNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes