Green’s Dictionary of Slang

1. a cheat’s accomplice, who lures victims into the game.

1827		J. Wight More Mornings in Bow St. 42: Every operator in this game is attended by certain of his friends called eggers and bonnnetters — the eggers to egg on the green ones to bet, by betting themsleves; and the bonnetters, to bonnet any green one who may happen to win, that is to say, to knock his hat over his eyes while the operator and the others bolt with the stakes .
1839		H. Brandon ‘Dict. Flash or Cant Lang.’ in ‘Ducange Anglicus’ Vulgar Tongue (1857) 4: Bonnetter – one who entices another to play.
1841		Comic Almanack Oct. 282: To call you Platt is both rude and raw, Just as if you were a man of straw, Or a twister of hair, or a man at a hell, / Playing the part of a Bonnetter well.
1859		Hotten Dict. of Modern Sl. etc. 10: Bonnetter one who induces another to gamble.
1859		Matsell Vocabulum 13: bonnetter One who entices another to play; or the fellow who takes the ‘flat’ in hand after the ‘roper in’ has introduced him to the house.
1864, 1867		Hotten Sl. Dict.
1873		Sl. Dict. 91: Bonnet, or Bonneter a gambling cheat. Sometimes called a ‘bearer-up.’ The bonnet plays as though he were a member of the general public, and by his good luck, or by the force of his example, induces others to venture their stakes.
1882		Sydney Sl. Dict. (2 edn) 2: Bounnetter [sic] - One who entices another to play.

2. a smashing blow on one’s hat.

1832		Satirist (London) 21 Oct. 341/2: When Sir .Charles Wetherell [...] had his hat thrust over his: eyes, which is vulgarly called a ‘bonnetter,’ he mumbled out that he would see them all [...] ignominiously damned.
1848		Flash Dict. in Sinks of London Laid Open.

3. one who smashes other people’s hats for amusement.

1852

Bell’s Life in Sydney 17 Jan. 3/1: Amongst the fashionable street amusements of tho metropolis, ‘bonneting’ [...] is the most in vogue [...] Having experienced such visitations; wo are enabled to state most confidently that ‘bonneteers’ are nuisances whioh ought to be put down with the strongest arm of the law.

4. (US) a stool pigeon.

1881

Trumble Sl. Dict. (1890).