Uncertain. Attested since the 1940s (in Berrey and Van den Bark’s 1942 American Thesaurus of Slang) or 30s. Semantic development from dyke (“ditch”) has been proposed, and some sources from the 1890s are said to record dyke as slang for "vulva" and hedge of the dyke as slang for "pubic hair", but Green's Dictionary of Slang says this is not found in connection to lesbianism and Dictionary.com considers a connection unlikely. Bull dyke / bulldike is attested earlier, in reference to women since at least the 1920s (the 29 July 1892 Decatur Daily Review in Illinois mentions a woman who "won the affections of Harvey Neal, alias 'Bulldyke'", whose gender is unclear); compare dike (“noun: well-dressed man; verb: be well dressed”)), bulldyker, and bulldyking, which are all attested earlier than bare dyke, e.g. in Parke's 1906 Human Sexuality, in the speech of Philadelphians, and backcountry black Americans. Compare bulldagger, attested since around the same time and used especially by black women. Other linguists suggested that bull dyke(r) referred to strong black women who dug dikes, or derived from bull + dick, perhaps in reference to black men. It has also been suggested dyke is a shortening of morphodyke, from morphodite, from hermaphrodite, but the derivation may go in the other direction instead, with morphodyke being a blend of morphodite with the already-extant word dyke.

bank • breakwater • Butch • cave • coastal dike • coastal dyke • dam • daughter of Sappho • den • Dike • embankment • floodwall • gay woman • hole • homosexual • lakelet • lesbian • levee • lezzy • mere • pit • pond • pool • puddle • sapphist • seawall • tidal dike • tidal dyke