"erectile organ of female mammals," 1610s, coined in Modern Latin from Late Greek kleitoris, a diminutive, but the exact sense intended by the coiners is uncertain. Perhaps from Greek kleiein "to sheathe," also "to shut," in reference to its being covered by the labia minora. The related Greek noun kleis has a secondary meaning "a key, a latch or hook (to close a door);" see close (v.), and compare slot (n.2).
Alternatively [Watkins], from Greek kleitys, a variant of klitys "side of a hill," from PIE *kleitor-, suffixed form of root *klei- "to lean," via with a sense of "little hill." Some ancient medical sources give a supposed Greek verb kleitoriazein "to touch or titillate lasciviously, to tickle" (compare German slang der Kitzler "clitoris," literally "the tickler"), but in this case the verb is likely from the anatomy.
As for the Greeks themselves, they seem to have called the thing nymphē, a figurative use, literally "bride, lovely young woman;" Beekes also has kystho-korone "clitoris," literally "crown of the vagina."
The anatomist Mateo Renaldo Colombo (1516-1559), professor at Padua, claimed to have discovered it ("De re anatomica," 1559, p. 243). He called it amor Veneris, vel dulcedo "the love or sweetness of Venus." It had been known earlier to women.