A sharper: perhaps from his preying upon any one he can lay hold of. Also a custom-house officer, or tide-waiter. Sharks; the first order of pickpockets. BOW-STREET TERM, A.D. 1785.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Shappo * SharpNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes