Green’s Dictionary of Slang

slug n.3

[supposed resemblances to the gastropod]

1. (US) a piece of nasal mucus.

[US]L. Weiner ‘Further Word-Lists – New Hampshire’ in DN IV:ii 154: slug, n. Ball of mucus in the nose.

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

[US]L. Kramer Faggots 321: Think schnitzel, schwantz, sewing machine, slug, spout, [etc.].
[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. [Internet] Slug. Penis. [Ibid.] Slug slewer. A penis watcher in the showers. A voyeur.
[US]T. Udo Vatican Bloodbath 50: We’re not really sure that he is yours anymore, are we Mr Floppy Slug?

In phrases

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

to masturbate.

[Aus]G. Seal Lingo 126: [...] tug the slug; beat the meat and punchin’ the truncheon, are all descriptions of male masturbation.