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 Up7’ [lyrics] Back out my ting and make man swim.

3. (UK Black/gang) as vtr, to stab a victim (to death).

1011 ‘No Hook’ [lyrics] Chef, chef swim, dip man down make him drown in his blood.

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.