Green’s Dictionary of Slang

1. (US gay) for a person using dentures to remove them before performing fellatio.

1971		E.E. Landy Underground Dict. (1972) gum it [...] Oral copulation done without teeth.
1988		H. Max Gay (S)language.

2. (US drugs) to hide a dose of methadone behind one’s teeth.

2021

T. Swerdlow Straight Dope [ebook] I was too curious about an old junkie on line at the methadone clinic as he ‘gummed’ his dose.