Green’s Dictionary of Slang

sweeten v.

also sweeten up

1. (UK Und.) to lure, to decoy, to swindle, to flatter.

[UK]Four for a Penny 8: The other meekly replies, Jack, be patient; ’tis a civil Gentleman, and I know will consider us : which species of Wheedling in Terms of their Art, is called Sweeten and Pinch.
[UK]B.E. Dict. Canting Crew n.p.: Sweeten c. to decoy, draw in, and Bite.
[UK]New Canting Dict. [as cit. c.1698].
[UK]Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[Scot]Life and Trial of James Mackcoull 25: The lawyer tried to sweeten him.
[UK]Leaves from Diary of Celebrated Burglar 83/2: Some time previous he had ‘sweetened’ Mag, the elder sister, so much that she consented to ‘pal in with him’.
[US]‘Mark Twain’ Life on the Mississippi (1914) 179: Yates called for his money [...] Stephen sweetened him up and put him off a week.

2. to bribe; to corrupt; thus sweetening n., bribery.

[Scot]D. Haggart Autobiog. 61: We sweetened the toping cove with plenty of budge.
[UK]Leaves from Diary of Celebrated Burglar 44/1: Through manifold apparent acts of kindness and plentiful ‘sweetening,’ he managed to ‘kid’ Teddy Grapes to let him have a run or two with him. [Ibid.] 86/1: Finally we both concluded that it was best to ‘sweeten’ the ‘cops’ by ‘slinging’ an occasional ‘doucer,’ and purchase their absence from the ‘drum’.
[US]A.H. Lewis Boss 133: ‘Didn’t you say as we came along that it would be proper to remunerate this officer for our encroachments upon his time?’ ‘Why, yes [...] I said that it might be a good idea to sweeten him.’.
[US]D. Lamson We Who Are About to Die 191: A tough beef can be beat, but not if you ain’t got no dough to put out to sweeten up the bulls and the ’cutors.
[US]Lait & Mortimer USA Confidential 139: They are still sweetening the kitty to keep it alive.
[UK]F. Norman in Punch 17 Mar. in Norman’s London (1969) 159: The fing is, me old mate, I’ve got ’em double sweetened up, in the flying squad.
[Aus]B. Humphries Traveller’s Tool 36: You’ll need to sweeten the appropriate authority with a real bundle.
[UK] in D. Campbell That Was Business, This Is Personal 18: He had already sweetened the filth.
[Aus]M.B. ‘Chopper’ Read Chopper From The Inside 93: Sweetening a Crown witness is a bloody must in my book.

3. to calm down, to assuage someone’s worries (cit. 1905 refers to holding off creditors); in criminal contexts, to calm a suspicious victim; thus sweetening n.

[UK]Leaves from Diary of Celebrated Burglar 51/2: This ‘sweetening’ succeeded, and she ‘slung’ him ‘half a bull’.
[UK]A. Binstead Mop Fair 145: Whatever was left in the weekly envelope after sweetening Topping and Spindler.
[UK]Wodehouse Right Ho, Jeeves 86: Softened up. Sweetened. Worked on. Preliminary spadework must be put in.
[UK]R.T. Hopkins Life and Death at the Old Bailey 59: If a person’s suspicions are aroused, then they try ‘to sweeten him,’ and ‘to keep him sweet’ until their object is accomplished.
[UK]J. Curtis Look Long Upon a Monkey 64: He’d have him sweetened up dead to rights in a minute, if that young mug Ray didn’t go and mess up the issue.
[UK](con. 1920s) J. Sparks Burglar to the Nobility 47: These police were quite decent [...] and began to sweeten us with tea and cigarettes.

4. to add alcohol to a non-alcoholic drink.

[US]J. Dixon Free To Love 145: Julia allowed Dan to ‘sweeten’ their ginger ale from his flask.

In phrases

sweeten the pot (v.) (also sweeten up) [poker jargon: to add money to a pot, to raise the betting]

to make a proposition more alluring, to improve a situation.

[US]Wood & Goddard Dict. Amer. Sl.
[US](con. 1960s) D. Goines Whoreson 235: She had sweetened the pot with the promise of twenty thousand dollars cash.
[US]D. DeLillo Names (1983) 64: The gamier the place, or the more ticklish politically, [...] they sweeten the pot, our New York masters.
[US]A. Rodriguez Spidertown (1994) 17: He tole me I should sweeten it [i.e. a bribe] up this time, so there’s some extra juice in there.