Subtle, acute, quick-witted; also a sharper or cheat, in opposition to a flat, dupe, or gull. Sharp’s the word and quick’s the motion with him; said of any one very attentive to his own interest, and apt to take all advantages. Sharp set; hungry.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Shark * SharperNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes