A pickpocket. Let us fork him; let us pick his pocket.—’The newest and most dexterous way, which is, to thrust the fingers strait, stiff, open, and very quick, into the pocket, and so closing them, hook what can be held between them.’ N.B. This was taken from a book written many years ago: doubtless the art of picking pockets, like all others, must have been much improved since that time.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Foreman of the Jury * Forlorn HopeNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes