Green’s Dictionary of Slang

tank v.1

[SE tank, a swimming pool, thus the boxer ‘takes a dive’]
[1920s+]

1. to abandon deliberately, to give up, poss. for illicit monetary gain, esp. in boxing.

2. to fail.

In compounds

tank fight (n.)

[1920s+] a contest in which one fighter has been bribed to lose.

tank job (n.) (also tank act)

[1930s] a corrupt sporting contest with a pre-arranged result.