Green’s Dictionary of Slang

working as a pickpocket.

1925		N. Lucas Autobiog. of a Thief 98: My pals went in for every known form of getting other people’s money [...] ‘dumming,’ ‘parlor jumping,’ ‘whizzing’ – [we] were experts in them all.
1936		H. Corey Farewell, Mr Gangster! 280: Slang used by English criminals [...] Whizzing – pocket-picking.
1941		V. Davis Phenomena in Crime 195: Picking pockets, or ‘whizzing’ as it is called.