Green’s Dictionary of Slang

1. a rogue who specializes in dropping something supposedly valuable where it will be found by a potential victim, who is either lured into a game or persuaded to buy the ‘valuable’, while the con-man claims that although they should, by rights, share the profits, he will sell his share and let the victim have the whole benefit.

1698		N. Ward ‘A Trip to Jamaica’ in Writings (1704) 154: I shall therefore Hussle them together as a Morefields Sweetner does Luck in a Bag.
c.1698		B.E. Dict. Canting Crew n.p.: Sweetners c. Guinea-Droppers, Cheats, Sharpers.
1708		J. Hall Memoirs (1714) 6: Sweetners, Such as drop Money before People, and taking out of Sight, inveigle a Man (after a hot Dispute with some of their Accomplices, who earnestly claim halves of what they find) into a Tavern they use, where they draw him into Cards, Dice, or Buckle and Thong, which they planted in some visible Place, and win all his Money: These sort of Vermine likewise go about the Countrey to cheat People of their Money by the Legerdemain Slight of Cups and Ball, and Luck in a Bag; this is a Function too that has not flourished since the late Act for Vagabonds.
1725		New Canting Dict. n.p.: sweetners Guinea-droppers, Cheats, Sharpers. The Fifty-first Order of Villains.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Sweetners, Guinea droppers, cheats, sharpers.
1809		G. Andrewes Dict. Sl. and Cant n.p.: Sweeteners guinea droppers.
1811		Lex. Balatronicum [as cit. 1785].
1828		Sussex Advertiser 14 Apr. 4/3: [We] soon passed a long string of gaggers, priggers, Adam Tylers, fancy coves, autum [sic] morts, gammoners, sweetners, uprightmen, bully huffs, lully priggers, star gazers, and coves of all sorts.
1835		G. Kent Modern Flash Dict. [as cit. 1809].
1848		Flash Dict. in Sinks of London Laid Open.

2. one who poses as an innocent player in order to ensnare a genuine innocent into playing a game in which he will invariably find himself the defrauded loser; cite 1859 refers to a supposed ‘sure thing’ tip.

1753		Thief-Catcher 16: There is another Class of these Sort of Gamblers, who are better distinguished by the Name of Sweetners, or Preachers of the Parson [...] they walk about, and soon find a pretence to pick up some wealthy Farmer, Gentleman, or a Clergyman, who has Money.
1859		Era (London) 20 Mar. 3/3: We had another sweetner for the Warwickshire Handicap. Yellow Rose being our selected ‘with a start’.
1885		Leeds Times 23 May 6/6: Prisoner [...] having been known as a ‘sweetener’ to card-sharpers for 12 or 13 years, was committed for two months.
1886		W. Newton Secrets of Tramp Life Revealed 11: Sometimes he will do it very slowly and you can see where it is, but the ‘jollier’ or ‘sweetener’ will bet him and win. This is to encourage you to stake more money, and when you do, this is the time he wins.

3. one who makes fake bids at an auction in order to drive up the prices.

1864, 1867, 1870		Hotten Sl. Dict. 251: SWEETENER, a person who runs up the prices of articles at an auction.
1896		G.F. Northall Warwickshire Word-Book 232: Sweetener. A person engaged on behalf of the seller to bid at a public sale or auction.