Green’s Dictionary of Slang

1. (US) a house player in a casino who entices genuine players to bet (and usu. lose) their money.

1901		C.R. Wooldridge Hands Up! 106: The booster meets the victim and conducts him to a saloon or byway and there the operator is found shaking three dice.
1909		W. Irwin Confessions of a Con Man 35: They [i.e. casinos] need boosters to stimulate interest and to keep the game running.
1935		A.J. Pollock Und. Speaks 11/2: Booster, an employee (decoy) who starts a game for a gambling house to lure the victims.
1947		D. Maurer ‘Argot of the Three-Shell Game’ in AS XXII:3 164: The boosters come in to keep the game going.

2. (US) any form of confederate working with a confidence trickster.

1905		Wash. Post 15 Jan. 4/5: He finds the same crowd of Keenes, Belmonts and Whitneys here. They are known by the police as the ‘boosters.’.
1913		Sun (NY) 27 July 40/1: It’s the old wire with a new twist. They have a phony poolroom [...] Mr Boob comes alonmg and the booster steers him in.
1914		Jackson & Hellyer Vocab. Criminal Sl. 18: booster [...] Used by confidential grafters. One who endorses a person, thing or action of immoral nature either by complimentary action or by moral support; a helper; a confederate.
1926		J. Black You Can’t Win (2000) 151: His ‘cappers,’ ‘boosters,’ and ‘shills’ fought with the yokels for a chance to get something for nothing and always beat them to the pieces of soap containing the money.
1960, 1975		Wentworth & Flexner DAS.
a.1985		‘The Open Book’ in G. Logsdon Whorehouse Bells Were Ringing (1995) 111: There’s boosters from Oklahoma, / and bastards from Arkansas, / But they’re just cotton pickers and tinhorn dice lickers / with not too much in their craw.

3. (W.I.) an aphrodisiac.

1995

Francis-Jackson Official Dancehall Dict. 5: Booster sexual stimulant, aphrodisiac.