1590s, "pertaining to the island of Lesbos," from Latin Lesbius, from Greek lesbios "of Lesbos," Greek island in northeastern Aegean Sea (the name originally may have meant "wooded"), 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. Before this, the principal figurative use (common in 17c.) was lesbian rule (c.1600) 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."
Cf. also tribadism.