1884 (by 1845 in German, 1824 in medical Latin), from Latin cunnus "vulva, female pudenda" (also, vulgarly, "a woman") + lingere "to lick" (from PIE root *leigh- "to lick"). Latin cunnus is of disputed origin, perhaps literally "gash, slit," from PIE *sker- (1) "to cut," or [Watkins] literally "sheath," from PIE *kut-no-, from root *(s)keu- "to cover, conceal."
The Latin properly would mean "one who licks a vulva," but it is used in English in reference to the action. The verb ought to be *cunnilingue. As an agent-noun, Fletcher has lick-twat (1656). Gordon Williams ["A Dictionary of Sexual Language and Imagery in Shakespearean and Stuart Literature," 1994] writes that Nicolas Chorier's 17c. "Satyra Sotadica" "relates how Gonsalvo of Cordova, as an old man, would lick his mistress's middle parts, which he called, with a geographical pun, going to Liguria" (from Latin ligurio "to lick").
Cunnilingus was a very familiar manifestation in classical times; ... it tends to be especially prevalent at all periods of high civilization. [Havelock Ellis, "Studies in the Psychology of Sex," 1905]
Dutch slang has a useful noun, de befborstel, to refer to the mustache specifically as a tool for stimulating the clitoris; probably from beffen "to stimulate the clitoris with the tongue."