Green’s Dictionary of Slang

1. (later use is US Und.) a ‘respectable’ figure who holds the profits of crime for a thief, one who holds money for a drug dealer (in case of their arrest) etc.

1608		Dekker Belman of London F3: In this Law they which play booty are the Banckers.
1895		Crowe Aus. Sl. Dict. 6: Banker, one who backs the game; the capitalist in any scheme.
1911		Wash. Post 14 May 13/3: The banker is becoming recognized as necessary to the thief [...] Another thing is that the thieves’ banker is likely to be more or less powerful politically.
1981		(con. c.1920) A. Harding in Samuel East End Und. 182: Posh Reed took over the job of paying the police – you know, the ‘banker’.
1986		C. Stroud Close Pursuit (1988) 319: Banker: Street slang for the person who receives and holds cash paid out for drugs.

2. (orig. gambling) a sure thing, something or someone on which one can depend, a safe bet (fig. and lit.) [bank on under bank n.¹ ].

1989		T. Blacker Fixx 244: The fourth — a divorcee, normally a banker on these occasions — actually offered me a bed in the spare room.
1999		K. Sampson Powder 132: Stick him on the cover and you’ll sell out. [...] We ain’t had a banker like Helmet in years.

3. (US und.) that figure in a numbers organization who deals with the actual cash, e.g. counting it, paying it out.

1932		Seabury Report 136: It is extremely profitable for the [policy] player to win, but very expensive for the ‘banker’, who sometimes arranges a fake raid to get out of paying his losses.
1940		E.F. Frazier Negro Youth 37: Even in crime, as for example in the ‘numbers’ racket, the white ‘bankers’ have maneuvered control into their hands.
1950		Goldin et al. DAUL 23/1: Banker. An operator of a policy-numbers game.
1973		P. Maas Serpico 165: next in the intricately structured racket is the pickup man, who brings the ‘work’—the betting slips—from various collectors to a controller. He in turn passes it on to a ‘banker,’ the money man.