An adept; a dab at any feat or exercise. Dab, quoth Dawkins, when he hit his wife on the a-se with a pound of butter.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1721 dictionary of canting and thieving slang.
to cutty-eye * daceNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes