Green’s Dictionary of Slang

soaker n.3

[? soak v.2 (1)]

(US prison) a warder.

[US]Goldin et al. DAUL 201/2: Soaker. (P.; mid-West, Central, and East Central U.S., some sections) A brutal prison guard. ‘Don’t fall (get arrested) around here. The soakers in lag (the jails) are murder (brutal).’.