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, characterized by erotic interest in other women" (in continuous use from 1890; the noun lesbianism from this sense is attested from 1870) and the noun, which is first recorded 1925.
Sappho's particular association with erotic love between women (with or without concurrent relations with men) dates to at least 1732 in writing in English, though the continuous use of lesbian and the modern words formed from it are from late 19c. The use of lesbian as a noun and an adjective in this sense seems to follow the same pattern.
In another Place the same commentator conjectures, that Myra is a Corruption of Myrrhina а famous Courtesan of Athens, who first practis'd and taught in that City Sappho's Manner and the Lesbian Gambols. ["Peregrine O Donald" (William King), "The Toast," 1732]
Before this, the principal figurative use of Lesbian was lesbian rule (c. 1600 and especially common in 17c.) a mason's rule of lead, of a type used in ancient times on Lesbos, which could be bent to fit the curves of a molding; hence, figuratively, "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"]
It also was used in English from 1775 in reference to wines from Lesbos. Though the specific "pertaining to female homosexuality" is recent, Lesbian had long before that a suggestion of "amatory, erotic," "From the reputed character of the inhabitants and the tone of their poetry" [Century Dictionary]. The island's erotic reputation was ancient; Greek had a verb lesbiazein "to imitate the Lesbians," which implied "sexual initiative and shamelessness" among women (especially fellatio), but not necessarily female homosexuality, and they did not differentiate such things the way we have.