Green’s Dictionary of Slang

slug n.3

[supposed resemblances to the gastropod]

1. [1910s] (US) a piece of nasal mucus.

2. [1940s+] (orig. Aus. navy) the penis; thus slug slewer, one who watches men in the shower.

In phrases

tug the slug (v.) [tug v. (2)]

[1990s+] to masturbate.