1591, from L. Lesbius, from Gk. lesbios "of Lesbos," Gk. island in northeastern Aegean Sea, home of Sappho, great lyric poet whose erotic and romantic verse embraced women as well as men, hence meaning "relating to homosexual relations between women" (1890; lesbianism in this sense is attested from 1870) and the noun, first recorded 1925. Slang variant lez, les is from 1929; lesbo first attested 1940. Before this, the principal fig. use (common in 17c.) was lesbian rule (1601) a mason's rule of lead, of a type used on Lesbos, which could be bent to fit the curves of a molding; hence, "pliant morality or judgment."

"And this is the nature of the equitable, a correction of law where it is defective owing to its universality. ... For when the thing is indefinite the rule also is indefinite, like the leaden rule used in making the Lesbian moulding; the rule adapts itself to the shape of the stone and is not rigid, and so too the decree is adapted to the facts." [Aristotle, "Nicomachean Ethics"]