Green’s Dictionary of Slang

a pickpocket.

1924		Townsville Daily Bulletin (Aus.) 14 June 13: A pickpocket is termed a ‘whizzer’.
1925		N. Lucas Autobiog. of a Thief 106: Paul and Bevis [...] were expert ‘whizzers,’ otherwise pickpockets.
1930		Eve. Herald (Dublin) 9 Dec. 4/6: ‘Whizzer,’ for pickpocket undoubtedly is a corruption of the Victorian ‘Gee whiz’, a reference to the slickness with which the pickpockets must work.
1936		Sunshine Advocate (Vic.) 11 Sept. 6/3: Pickpockets are known to their associates are whizzers, dips or hooks.
1938		F.D. Sharpe Sharpe of the Flying Squad 16: Moisher the Gonnof was a wizzer. [Ibid.] 245: These wizzers get up to all sorts of dodges.
1946		S. Jackson Indiscreet Guide to Soho 24: The pickpocket, or ‘whizzer’ [...] came running out of Frith Street into Soho Square.
1957		P. Beveridge Inside the C.I.D. 200: Whizzers Pickpockets.
1970		R. Fabian Anatomy of Crime 194: Whizzer: Expert pickpocket.
1981		(con. c.1906) A. Harding in Samuel East End Und. 77: I said to him ‘What are you at?’ and he said, ‘I’m a whizzer’ (pickpocket).