Green’s Dictionary of Slang

1. a reward of £40.

1800		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.
1827		Egan Anecdotes of the Turf, the Chase etc. 44: [note] She’ll surely turn snitch for the forty.
1833		‘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].

1882		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’ .
1898		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.
1900–10		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.
1910		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.
1910		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.
1955		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.

1985		T.R. Houser Central Sl. 23: forty [...] ‘I cleaned-up the shit and the dude bought me the forty.’.
1993		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.’.
1995		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.
2000		W. Shaw Westsiders 13: A ‘four-oh’ is shorthand for a 40-ounce bottle of beer, usually a brand like O.E., Old English.
2001		J. Stahl Plainclothes Naked (2002) 114: He pimp-walked towards her, swigging his forty.
2001		(con. 1990s) in J. Miller One of the Guys 86: ‘Everybody brings bud and forties. Sit there and get fucked up’.
2002		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 .
2008		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).

2004

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’.