1887, from Latin cunnus "vulva" (see cunt) + lingere "to lick" (see lick (v.)). The Latin properly would mean "one who licks a vulva," but it is used in English in reference to the action, not the actor. The verb ought to be *cunnilingue.
Cunnilingus was a very familiar manifestation in classical times; ... it tends to be especially prevalent at all periods of high civilization. [Havelock Ellis, 1905]
cunnilingus cun·ni·lin·gus (kŭn'ə-lĭng'gəs)
Oral stimulation of the clitoris or vulva.