Green’s Dictionary of Slang

1. in concrete senses, a cheating device in gambling (or in any crooked form of sideshow), orig. a small hook set in a ring used by a card-sharp.

1859		Matsell Vocabulum 112: gaff The gaff is a ring worn on the fore-finger of the dealer. It has a sharp point on the inner side, and the gambler, when dealing from a two-card box, can deal out the card he chooses.
1890–1904		Farmer & Henley Sl. and Its Analogues III 97/1: Gaff 4. (old sharpers’). — A ring worn by the dealer [From gaffe = a hook].
1927		K. Nicholson Barker 149: Gaff – A dishonest gaming device.
1938		(con. 1820s+) H. Asbury Sucker’s Progress 12: For years there was a steady stream of dishonest appliances bearing such names as gaff [...] the gaff was a small instrument shaped like a shoemaker’s awl and worn attached to a finger ring.
1949		Monteleone Criminal Sl. (rev. edn).
1953		W.L. Alderson ‘Carnie Talk’ in AS XXVIII:2 116: gaff, v. and n. General term for illegitimate control of gambling wheel or other device. ‘To gaff the joint’ or ‘put on the gaff,’ to add such a device.
1953		F. Brown Madball (2019) 71: ‘Joe Linder’ll have to show you the gaff on that rope tie trick’.
1961		J. Scarne Complete Guide to Gambling 378: The croupier in a juice joint (gambling house which has an electromagnetic wheel) can successfully operate the gaff only when the wheel is spinning very slowly.
1968		T. Thackrey Gambling Secrets of Nick The Greek 240: When you think such a gaff is being worked on you [...] it’s a good idea not to press the matter too hard.
1971		(con. 1920s) J. Brown Monkey Off My Back (1972) 16: My employers operated a ‘flat store’ — a game of chance that could be controlled with a ‘gaff’ or ‘break’.
2012		L. Berney Whiplash River [ebook] ‘“We’re good with the gaff,’ Shake said. ‘It only cost me a hundred bucks’.
2012		W. Keyser ‘Carny Lingo’ in http://goodmagic.com 🌐 Gaff — The mechanism by which a game is secretly controlled or ‘faked’.

2. (US Und.) a fraud, a racket.

1922		Broadway Brevities Aug. 38/2: And isn’t the ‘prominent shareholder’ sitting on pins for fear Corrie may get wise to the whole gaff? [i.e. an adulterous affair].
1936		J. Curtis Gilt Kid 21: I got a good gaff. I could do it on my own if I wanted to, but I likes to have a bit of company when I goes on a job.
a.1967		‘Sporting Life’ in D. Wepman et al. Life (1976) 162: There are fellows who laugh when they use the gaff / To take a sucker’s dough.
2014		D.B. Flowers Bangs 37: [H]eand Brother O’Leary still enjoyed pulling gaffs together, both for pleasure and profit. Brother had his own favorite gaff, a con he had perfected over the course of several years.

3. (US Und.) the place – a fake ‘bookmaker’s’ or ‘stockbroker’s office’ – in which a confidence trick is carried out.

1940

D. Maurer Big Con 105: It is sport of a high order to play them successfully to the gaff.

4. (US) in pl., crooked dice.

1950

(con. 1940s+) D. Maurer ‘Lang. of the Professional Dice Gambler’ in Lang. Und. (1981) 186/1: gaffs: Crooked dice of any kind.

5. in fig. use of sense 1, a gimmick, a hidden trick.

1946		T. Thursday ‘Raw, Medium, and Well Done’ in Blue Ribbon Western June 🌐 ‘What’s the gaff?’ I says. ‘Tell papa!’.
1953		A.J. Liebling Honest Rainmaker (1991) 77: The old-time bank (faro) play who had to go against the gaff (the gimmick that put him at the dealer’s mercy).
1967		L. Bruce Essential Lenny Bruce 139: The gaff is that they’re going to help young talent.