Green’s Dictionary of Slang

1. to gamble, esp. to toss coins; thus gaffing school, a group of gamblers .

1811		Lex. Balatronicum n.p.: To Gaff. To game by tossing up halfpence.
1812		Vaux Vocab. of the Flash Lang.
1819		P. Egan Key to the Picture of the Fancy going to a Fight 28: Numerous Piemen [...] will either sell their articles, or gaff for them.
1822		Tom And Jerry; Musical Extravaganza 53: Gaffing, low gambling.
1828		G. Smeeton Doings in London 39: When he gaffs, he contrives to have both halfpence under his hand [...] This ingenious fellow has often won 200 or 300 sovereigns [...] by gaffing.
1831		Satirist (London) 20 Nov. 262/1: He has a halfpenny with two heads, and a halfpenny with two tails. When he gaffs, he contrives to have both halfpence vnder his hand.
1841		Bell’s Life in London 22 Apr. 4/4: We observed sundry groups engaged in [...] gaffing.
1843		Satirist & Sporting Chron. (Sydney) 1 Apr. 1/1: It matters little whether Johnny R—s is the son of the Cockatoo, or not, he is now a respectable young man, although slightly given to Gaffing.
1848		Flash Dict. in Sinks of London Laid Open 108: Gaffing, tossing with the pie man.
1860		R. Nicholson Rogue’s Progress (1966) 75: ‘gaffing,’ i.e. tossing [...] was at that time [i.e. 1830s] the only game played by sporting men, such as prize-fighters, police-officers, and many of the highest of them were inveterate gamblers.
1888		‘Rolf Boldrewood’ Robbery Under Arms (1922) 17: Catch him gaffing! — no, not for a sixpence.
1901		Capricorn (Rockhampton, Qld) 31 Aug. n.p.: I’ve got a cheque here: I got it at ’gaffing’ at Duaringa.
1902		‘Dads Wayback’ in Sun. Times (Sydney) 16 Nov. 5/5: ‘[T]hey jines er gaffin’ school an’ heads ’em, or plays two up an’ ther like o’ that’.
1904		R. Free Seven Years Hard 122: Perhaps the reader may not be aware that [...] rum is ‘Nelson's blood’; that ‘gaffing’ is gambling ; ‘lumping,’ pawning.
1911		A. Wright Gamblers’ Gold (1931) 109: I blued me cheque drinkin’ and gaffin’.

2. (US) to cheat, to rig, to fix.

1903		A.H. Lewis Boss 151: The question now is, how to gie th’ Chief th’ gaff, an’ gaff him deep an’ good.
1916		H.L. Wilson Somewhere in Red Gap 77: That’s what really gaffed me the worst!
1938		J. Digges Bowleg Bill in Botkin (1944) 21–28: Well, you got to know how to gaff ’em in. Hoss-mackereling is no business for a green hand.
1946		J.E. Dadswell Hey, Sucker 103: A G-joint [...] is a concession that has a controlled device. When a concession is so operated it is said to be ‘gaffed’.
1956		H. Gold Man Who Was Not With It (1965) 135: ‘Look, you can gaff it for the extra per cent. You’re in charge. You can even cheat me.’ [...] ‘I just want to play you straight and ungaffed at fifty for fifty.’.
1963		A.S. Fleischman Venetian Blonde (2006) 233: It was his body that you gaffed to pass for Jamie’s.
1968		T. Thackrey Gambling Secrets of Nick The Greek 126: ‘Gaffed,’ he said. ‘It’s a bitter thing to admit – but not one illegal Faro game in Los Angeles is on the level.’.
1975		J. Roe The Same Old Grind 37: We all got arrest, them for gaffing, and me for lewd.
2001		‘Randy Everhard’ Tattoo of a Naked Lady 6: I could gaff the joint to make it impossible to win. But where’s the fun in that?

3. (US) to make a game crooked or dishonest, typically to tamper with a fruit machine or roulette wheel.

1949		Monteleone Criminal Sl. (rev. edn).
1967		‘Iceberg Slim’ Trick Baby (1996) 93: Blue is gonna gaff that wheel on your number and heave you a heavy cop.
1975		H. Ellison ‘Pretty Maggie Moneyeyes’ in Deathbird Stories (1978) 129: ‘Nobody’s been spooning it.’ [...] ‘Gaffing. That’s what he meant. Spooning’s another word for it. Some guys use a little piece of plastic, or a wire, shove it down through the escalator, it kicks the machine.’.
2012		W. Keyser ‘Carny Lingo’ in http://goodmagic.com 🌐 ‘The game is gaffed’ is sometimes expressed as ‘the game is G’d’.