Green’s Dictionary of Slang

1. a gambling cheat, who poses as a normal player, thus luring the victim to join the game, but who, as the play proceeds, begins cheating in favour of the bank or house; also used in non-gambling situations.

1828		‘A Flat Enlightened’ Life in the West I 212: These persons also pick up flats at places of refreshment, &c. and bring them under their arms to the tables to which they are attached. These bonnets are also very useful in another way [etc].
1832		Satirist (London) 13 May 159/1: The bonnet is some poor or distressed sharper, who is out at elbows, and who is dressed by his employers, for the purpose of attending the table, and playing as an individual totally unconnected with Hui establishment.
1839		Manchester Courier 29 June 2/3: One by one the confederates, or bonnets, came cautiously up and having arranged themselves around the table, the sport began.
1848		Era (London) 4 June 4/2: With a nobby lot of bonnets, and a swellish kind of kid, / You Loadston’d us, and cleverly your true intentions hid.
1853		G.J. Whyte-Melville Digby Grand (1890) 305: I began to think my military friend was ‘a bonnet’ — one of those harpies employed by gambling-house keepers to enhance temptation by the influence of example, and generally selected for their respectable and innocent appearance.
1859		Bell’s Life in Tasmania 19 July 4/2: [A] man whom I afterwards ascertained to be a ‘bonnet,’ and who upon all occasions took the parts of gentlemen's grooms.
1860		R. Nicholson Rogue’s Progress (1966) 117: The distinguished position of being a hell-keeper’s tout, a picker-up or bonnet.
1861		H. Mayhew London Labour and London Poor IV 25: the dependents of cheats are [...] 2. ‘Bonnets,’ or accomplices of Gamblers.
1870		Hotten Sl. Dict.
1880		W. Hooe Sharping London 34: Bonnet, a confederate in gambling.
1881		Herald (Melbourne) 6 Aug. 2/6: They Say [...] A card-sharper got into the carriage with us and the ‘bonnet’ joined us.
1898		Binstead & Wells A Pink ’Un and a Pelican 198: In less than twenty minutes they’d skinned ‘The Lout,’ who, after all, was more of a bonnet than a spieler.
1907		Lone Hand (Sydney) Aug. 439/2: If the flat is disinclined, sharp No. 3, the ‘bonnet’ or ‘buttoner’ [...] steps forward and picks the pea.

2. a pretext or pretence, esp. as the legitimate job behind which a thief hides their true occupation, e.g. a newspaper seller or porter.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 228: bonnet: a concealment, pretext, or pretence; an ostensible manner of accounting for what you really mean to conceal; as a man who actually lives by depredation, will still outwardly follow some honest employment, as a clerk, porter, newsman, &c. By this system of policy, he is said to have a good bonnet if he happens to get boned; and, in a doubtful case, is commonly discharged on the score of having a good character.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1867		Sportsman 19 Feb. 2/1: Notes on News [...] Celibacy [is] nothing more than a ‘bonnet’ for the desolate ‘wallflowers’ who, crying ‘any, Lord, any!’ are waiting for the ‘coming man’.
1873		Sl. Dict. 91: Bonnet [...] a pretence, or make-believe.

3. a shop tout or one who encourages sales for a street vendor by praising the goods.

1848		‘Epistle from Joe Muggins’s Dog’ in Era (London) 4 June 4/1: Poor John Davis, why don’t you take to an honest calling, and not turn bonnet for a tailor?
1876		C. Hindley Life and Adventures of a Cheap Jack 217: We bid or praised up his goods: in fact, often acted as ‘puffers’ or ‘bonnets’ to give him a leg up.

4. a sham bidder at auctions who works to drive up the price.

1860		Cork Examiner 8 Jan. 4/6: The Mock Dutch Auction. Little Johnny Woburn, Ben Dizzy, the Derby Slogger, and Jack the Quaker, well known touts and ‘bonnets’ [...] sellin’ on the Dutch principle.
1864, 1867, 1870		Hotten Sl. Dict.