Green’s Dictionary of Slang

1. used of people, criminal, e.g. pickpockets, con-men.

1852

Era 4 jan. 3/3: Thares beene nothink doin at ther Korner lately, but ‘gaff’ peepel is restin abit [...] ther swells iz oute ov towne, and ther sharpe handes dont see enney pull in eeting each other up.

2. (US gambling) rigged.

1928		implied in gaff joint
1931		Irwin Amer. Tramp and Und. Sl. 82: Gaff Wheel.—A gambling wheel controlled by the foot of the operator, who by pressing upon a board built into the platform upon which he is standing is able to stop the wheel at any desired point.
1949		Monteleone Criminal Sl. (rev. edn) 93: gaff wheel A crooked gambling wheel controlled by the operator.
1953		A.J. Liebling Honest Rainmaker (1991) 113: They learned of the gaff roulette game below decks.
1962		Ragen & Finston World’s Toughest Prison 800: gaff wheel – A gambling wheel controlled by the foot of the operator.