Green’s Dictionary of Slang

1. a prize-fighter who has agreed to accept cash in return for losing a fight.

1947		B. Schulberg Harder They Fall (1971) 217: Shove his pride [...] You think he’s the only tanker in Las Vegas.
1955		R. Graziano Somebody Up There Likes Me (1956) 243: You know – tankers, fixed fights [...] You’ve gone in the tank just like everybody else has, sometime or other, haven’t you Rocky?
1955		B. Schulberg On the Waterfront (1964) 263: One lousy rubber-lipped ex-tanker who’s walking on his heels.

2. in fig. use, a second rate person.

1929

A.J. Barr Let Tomorrow Come 8: The rap is the label that shows you big-time or a tanker.