Green’s Dictionary of Slang

(US) fellatio, esp. when the testicles are held in the mouth and the woman hums; thus in fig. use.

1971		E.E. Landy Underground Dict. (1972).
1991		Gazette (Montreal, CA) 11 Aug. 23/2: There is even a list of more than 100 types of kisses, including the [...] French Hummer.
1995		S. Moore In The Cut 63: I have many more words for the dictionary [...] hummer, sexual act of holding testicles in the mouth and humming; also hum-job (Bronx word).
2001		Herz & Steinberg Amer. Pie 2 [film script] I got laid 23 times this year, and I’m not counting the hummer I got in the library stacks, baby.
2006		C. Hiaasen Nature Girl 48: The best damn hummers you ever had in your life.
2016		‘Chelsea G. Summers’ in hazlitt.net 31 Mar. 🌐 ‘Milkshake,’ ‘skull-fuck,’ ‘hummer,’ or ‘head’ all name it; likewise, you might suck a dick—or you might enjoy getting your cock sucked, or both.