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from Latin labia, plural of labium "lip" (see lip (n.)). Specifically of the folds on either side of the vulva from 1630s (labia pudendi).
labium la·bi·um (lā'bē-əm)
n. pl la·bi·a (-bē-ə)
A lip or lip-shaped anatomical structure.
Any of four folds of tissue of the female external genitalia.