FILE CLOY, or BUNGNIPPER. A pick pocket. To file; to rob or cheat. The file, or bungnipper, goes generally in company with two assistants, the adam tiler, and another called the bulk or bulker, whose business it is to jostle the person they intend to rob, and push him against the wall, while the file picks his pocket, and gives the booty to the adam tiler, who scours off with it. Cant.
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.
Filch * FinNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes