Green’s Dictionary of Slang

1. (UK Und.) a constable; latterly a police officer.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Bus napper a Constabkle. Cant.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1811		Lex. Balatronicum.
1812–82		B.M. Carew Life and Adventures.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1830		Bell’s Life in London 17 Oct. 3/1: A mill of minor description came off on Monday last [...] The thing was kept quite snug, lest the Beaks [...] or their leary ‘buz-nappers’ should obtrude their unwelcome nobs on the occasion.
c.1850		Duncombe New and Improved Flash Dict.
1859		Matsell Vocabulum.
1881		Trumble Sl. Dict. (1890).
1895		Crowe Aus. Sl. Dict. 13: Busnapper, a constable.

2. a young pickpocket.

1754		implied in bus-napper’s kinchin
1781		G. Parker View of Society II 173: This Rig is generally executed by a young fry of boys, who are first pick’d up in the purlieus of St. Giles’s, [...] they are put into a room, in which there are figures dressed up like a man and a woman, with bells in every pocket for the young ones to practice on [...] If they can make the dive, take book, handkerchief, or purse, without making the bells speak in the execution of this business, then they are qualified to take their degrees, go out on the leer the next darkey, and follow the profession of a Buz-napper.
1864, 1867, 1870		Hotten Sl. Dict. 91: BUZ-NAPPER, a young pickpocket.
1873		Sl. Dict.
1935		A.J. Pollock Und. Speaks 17/1: Buz-napper, a pickpocket. Buz-napper’s bloke, a dressed dummy on which youthful thieves practice and are taught to pick pockets.