Green’s Dictionary of Slang

swim v.

1. (US prison) to conform to prison society.

[US]Cardozo-Freeman & Deloreme Joint n.p.: The ‘keeper’ does not prepare the fish ahead of time for the ordeal that awaits them in the tank . . . new fish who do not learn immediately how to swim will undergo a devastating initiation rite [R].

2. (UK Black/gang) to be bleeding after being stabbed.

1011 ‘Next Up?’ [lyrics] Back out my ting and make man swim.

3. (UK Black/gang) as vtr, to stab a victim (to death), thus swimmer, a victim of knife crime.

1011 ‘No Hook’ [lyrics] Chef, chef swim, dip man down make him drown in his blood.
Loski ‘Cool Kid’ [lyrics] Splash, what a chinger / Turn them swimmer.

In phrases

swim in golden grease (v.) (also swim in golden lard)

to be offered and to take an abundance of bribes.

[UK]Jonson Volpone I i: When you do come to swim in golden lard, Up to the arms in honey.
[UK]Farmer & Henley Sl. and Its Analogues 47/1: to swim in golden grease (oil, lard, etc.), verb. phr. (old).— To ‘roll’ in bribes.