Green’s Dictionary of Slang

1. an old, toothless pit-bull or other fighting dog.

1876		J. Greenwood Low-Life Deeps 19: The first practice a fighting pup had was with ‘a good old gummer’.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.

2. a toothless person.

1908		Sporting Times 20 June 1/4: ‘Be sure that my steak is a tender one, as my teeth are nearly all gone.’ And the waiter stepped over to the kitchen lift and shouted up it: ‘Cook! Yearling rump for a gummer!’.
1911		Bisbee Dly Rev. (AZ) 12 Dec. 4/5: Walter yells in strident tones: ‘Cowmeat for a gummer’.
1938		J.B. Booth Sporting Times 89: Cook! Yearling rump for a gummer!

3. (US) an act of fellatio from an old, toothless person.

1997–2002

Alt. Eng. Dict. 🌐 gummer (noun) blow job from an old (toothless) person derived from ‘hummer’ with alteration concerning lack of teeth in mouth.