Green’s Dictionary of Slang

1. (US) a piece of nasal mucus.

1914

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 slew, to assess other men’s penises; slug slewer, one who watches other men in the shower.

1978		L. Kramer Faggots 321: Think schnitzel, schwantz, sewing machine, slug, spout, [etc.].
1984		B. Ellem Doing Time 197: slug slewer: a penis watcher in the showers.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Slug. Penis. [Ibid.] Slug slewer. A penis watcher in the showers. A voyeur.
2000		T. Udo Vatican Bloodbath 50: We’re not really sure that he is yours anymore, are we Mr Floppy Slug?
2006		B. Matthews Intractable [ebook] Peter was a notorious cat and when an escort arrived at the jail he would wait in the showers and slug slew the new arrivals.