Green’s Dictionary of Slang

1. a pickpocket; thus cannon-coppers, the pickpocket squad.

1902		Wash. Times (DC) 14 Sept. 10/3: Cannon or Dip — A pickpocket.
1910		J. Sullivan ‘Criminal Sl.’ in Amer. Law Rev. LII (1918) 890: A pickpocket is frequently called a ‘dip’ and in Western states a ‘cannon.’.
1924		G. Henderson Keys to Crookdom 42: Vollmer divides the dip into two classifications – one being the ‘lone wolf’ and the other the ‘gun-mob’ or ‘cannon’ type.
1924		in L.A. Times 30 Jan. A3: A ‘mob’ can ‘beat a pap’ to the ‘leather’ and get away with it with the ordinary ‘fuzz’ lookin’ on. But it’s a twenty-to-one shot when the ‘cannon copper’s are wise.
1933		‘Goat’ Laven Rough Stuff 17: I’d always wanted to try my skill as a cannon (pickpocket).
1937		(con. 1905–25) E.H. Sutherland Professional Thief (1956) 44: The term ‘cannon’ is used to designate the pickpocket and also the racket of picking pockets. The theory of the origin of this term is that the pickpocket some centuries ago was called a gonnif, which is the Jewish word for thief. This term was then abbreviated to ‘gun’; later someone in a moment of smartness referred to a pickpocket as a ‘cannon’ to designate a big gun, and the term ‘cannon’ then became general. The term ‘gun’ is still used to refer to pickpockets, and the female pickpocket who operates upon men is called a ‘gun-moll.’.
1937		‘Boxcar Bertha’ Sister of the Road (1975) 113: Most of the cannons (pickpockets) were married.
1940		D. Maurer Big Con 202: Those are just some old cannons.
1949		Monteleone Criminal Sl. (rev. edn) 43: cannon copper A pickpocket detective.
1952		C. Hamilton Men of the Und. 79: The modern cannon grifts on the subway.
1955		Q. Reynolds Police Headquarters (1956) 235: The term ‘live cannon’ is one given to a pickpocket skilful enough to remove a wallet from the back pocket of an alert and intelligent victim.
1969		‘Iceberg Slim’ Pimp 86: A ‘cannon’ with a tired horse face took the vacant stool.
2001		‘Randy Everhard’ Tattoo of a Naked Lady 124: I know he don’t appreciate cannons in his joint.

2. the act of pickpocketing.

1937		(con. 1905–25) E.H. Sutherland Professional Thief (1956) 27: The cannon is operated very rarely single-handed. [Ibid.] 44: The term ‘cannon’ is used to designate the pickpocket and also the racket of picking pockets.
1952		C. Hamilton Men of the Und. 320: Cannon, [...] 2. The act of picking pockets.