Green’s Dictionary of Slang

1. (orig. US, also bank) to provide financial backing for a project, legal or otherwise.

1929		J. Lait Broadway Melody 50: They pooled something more than a quarter of a million dollars to bank roll ‘Broadway Melody’.
1937		(con. WW1) E.C. Parsons Great Adventure 90: His squadron activities were numerous. Banking a roulette game, playing poker, keeping up with his drinking and incidentally taking out patrols .
1944		D. Runyon Runyon à la Carte 92: I am not able to bankroll you to a very large start.
1958		Kerouac letter July in Charters II (1999) 136: Anonymous angel bankrolled it, probably Max Gordon owner of the Blue Angel.
1969		‘Iceberg Slim’ Pimp 39: He would bank roll our venture.
1974		V.E. Smith Jones Men 102: He’s trying to turn over some of the stuff [...] to bankroll the deal.
1986		C. Hiaasen Tourist Season (1987) 119: It was bingo money that bankrolled Las Noches.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 175: I know he’s bankrolled a lot of movies under the table.
1990		S. Morgan Homeboy 289: I aint bankin F Stop no more.
1995		G. Small Ruthless 243: Once the Spanglers’ lawlessness was sanctioned – bankrolled and armed – by megalomaniac politicos.
2000		Guardian 7 Jan. 18: Playboy Taki risks being hounded by Mohammed Al Fayed after bankrolling loser Neil Hamilton’s libel trial.
2006		G. Pelecanos Night Gardener 113: How we known this [...] fella ain’t bein bankrolled by someone with power?

2. (US Und.) to take a victim’s money in a confidence trick where he is allowed to win and lose, but always loses more than he wins.

1940

D. Maurer Big Con 289: To bankroll. 1. For the insideman to finance an outside man with expense-money. 2. to take a victim’s money allowing him to win and lose, always losing more than he wins.