deep throat, n. — Green’s Dictionary of Slang

deep throat n.

[the term was popularized by the ‘art porn’ film Deep Throat (1973) starring Linda Lovelace; it was also used as the nickname of the otherwise anonymous source who helped journalists investigate the Watergate Affair (1972–4)]

deep fellatio, in which the penis is taken not simply into the mouth, but down the throat; cite 2017 ref. to cunnilingus.

1973		Deep Throat [film title].
1979		Maledicta III:2 218: In New York one x-rated (euphemistically, adult) cinema offered on one recent bill French Teens, French Throat (a rip-off of Deep Throat, a porno title that surfaced later in Watergate politics), and French Classmates – three at one blow, as it were.
2000		M. Manning Get Your Cock Out 84: Dandy nearly choked. Her Dad hadn’t taught her the deep throat technique.
2017		Young M.A. ‘OOOUUU’ 🎵 [of cunnilingus] Baby gave me head: that’s a low blow / Damn, she make me weak when she deep throat.