Green’s Dictionary of Slang

soak n.1

[soak v.1 (1)]

1. [late 18C+] a drunkard, usu. with old; note earlier soaker n.1 (1)

2. attrib. use of sense 1.

3. [mid-19C] (US) a drink.

4. [mid-19C+] a heavy drinking session.

5. [1900s–50s] a despised person.

In compounds

soakapee (n.) [SE soak + pee n.1 (1); such drunkards may well foul their clothes]

[1940s] (W.I.) a habitual drunkard, an alcoholic.

soakpot (n.)

[18C] a drunkard.

In phrases

get a soak on (v.)

[late 19C] (US) to drink heavily.