Green’s Dictionary of Slang

forty n.

1. a reward of £40.

[UK]Sporting Mag. Apr. XVI 26/1: If anything is done by scampsmen on the Fulham road, send the traps to pull up Bounce and Blunderbuss, two forties at least.
[UK]Egan Anecdotes of the Turf, the Chase etc. 44: [note] She’ll surely turn snitch for the forty.
[UK]‘The City Youth’ in Out-and-Outer in Spedding & Watt (eds) Bawdy Songbooks (2011) IV 140: The traps they buff it home the forty for to chop.

2. (Aus.) a crook, a confidence trickster, a rascal, a layabout youth; often in pl. [a Sydney gang of the mid-19C; poss. called the Forty Thieves, although that name may have been a subsequent journalistic invention].

[Aus]Sydney Sl. Dict. 8/2: The Forties, the worst types of ‘the talent’ who get up rows in a mob, [...] and sometimes assault and rob, either in barrooms or the streets. Name originated with sa gang in Sydney under ‘Dixon the dog hanger,’ ‘King of the Forties’ .
[UK]H. Macilwaine Dinkinbar 53: If a man happens to be born a forty, he finds decent company in England a bit too hot to hold him.
[Aus]Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 72: FORTIES: larrikins: gangs of men and youths who congregate about streets and public places, often getting into crime or mischief. [...] Forties in Sydney preceded Larrikins as a name for the street prowlers, gamblers thieves etc.
[Aus]C.E.W. Bean On the Wool Track 226: A few sharpers [...] often get into a big shed, and get a ‘school’ going—a nightly gamble. They are regularly called ‘forties’—the forty thieves—and they sometimes make a pile out of young shearers.
[Aus]Bulletin (Sydney) 13 Oct. Red Page/2: By the way, Bean mentions ‘forties’ as a common term for bush spielers. The note on its use in MacIlwaine’s Dinkinbar brought information from several correspondents, who are hereby thanked.
[Aus]N. Pulliam I Travelled a Lonely Land (1957) 233/1: fortie – a crook, double-crosser.

3. (US black, also 40, forty ounce, four-oh) a 40oz. (1-litre) bottle of beer.

[US]T.R. Houser Central Sl. 23: forty [...] ‘I cleaned-up the shit and the dude bought me the forty.’.
[UK]J. Mowry Six Out Seven (1994) 150: He tilted the bottle way back and gulped [...] Hobbes’s laugh came from beside him. ‘That the way to slam a forty, man.’.
Bone Thugs-N-Harmony ‘1st Of Tha Month’ 🎵 E. 1999 Eternal [album] Cuz it’s the 1st of tha month and now we smokin’, chokin’, rollin’ blunts / And sippin’ on 40 ounces thuggin’ come come we got the blessed rum.
[US]W. Shaw Westsiders 13: A ‘four-oh’ is shorthand for a 40-ounce bottle of beer, usually a brand like O.E., Old English.
[US]J. Stahl Plainclothes Naked (2002) 114: He pimp-walked towards her, swigging his forty.
[US](con. 1990s) in J. Miller One of the Guys 86: ‘Everybody brings bud and forties. Sit there and get fucked up’.
[US]Simon & Burns ‘The Target’ Wire ep. 1 [TV script] Yo’ ass be on the bottom end of Vine Street suckin’ on a ofrty yellin’ 5-0 .
[US]R. Price Lush Life 24: Just the Old Heads out there sippin’ forties .

4. (US drugs) a half-gram of cocaine (then sold for $50).

[US]J.A. Juarez Brotherhood of Corruption 227: ‘Can you give me change for this hundo? I’m gonna score a forty and need some small bills’.

In compounds

forty dog (n.) (also dog) [dog n.7 ]

(US black) a 40oz (1-litre) bottle of beer.

[US]Beastie Boys ‘Brass Monkey’ 🎵 I’ll down a 40 dog in a single gulp.
[UK]J. Mowry Six Out Seven (1994) 435: Go snag yourself a forty-dog, Sabby.
[US]Ebonics Primer at 🌐 dogs Definition: 1 or more bottles of 40oz malt liquer. Example: Yo money, go grab some dogs down at da corner store.
Sean Slaughter ‘Street Corner Catz’ 🎵 Played the game, pounds of trees, forty dogs and robberies.