FORK,
a Pick-pocket. Lets Fork him; Let us pick that Man's Pocket. It is done by thrusting the Fingers, strait, stiff, open and very quick into the Pocket, and so closing them, hook what can be held between them.
Fork in the 1811 Vulgar Tongue
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).