Dyke
Middle English (denoting a trench or ditch): from Old Norse dík, related to ditch. dyke1 (sense 1 of the noun) has been influenced by Middle Low German dīk ‘dam’ and Middle Dutch dijc ‘ditch, dam’.
wiktionary
A variant of dike, from Northern Middle English dik and dike(“ ditch”), from Old Norse díki(“ ditch”). Influenced by Middle Dutch dijc(“ ditch; dam”) and Middle Low German dīk(“ dam”). [1]See also ditch.
Uncertain. Attested since the 1940s (in Berrey and Van den Bark’s 1942 American Thesaurus of Slang) [2] or 1930s. [3]
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", [4] [5] but Green's Dictionary of Slang says dyke in the latter phrase had no reference to lesbianism and Dictionary.com considers a connection unlikely. [6]
Bull dyke / bulldike is attested earlier, in reference to women since at least the 1920s [6] [4] (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), [7] [8] and bulldyker (and the practice of bulldyking) are also attested earlier, e.g. in Parke's 1906 Human Sexuality, in the speech of Philadelphians, [9] and backcountry American blacks. [4] Compare bulldagger, attested since around the same time [6] and used especially by black women. [10] [11]
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. [4] [12]
etymonline
dyke (n.)
"a lesbian," especially one considered tough, mannish, or aggressive, 1931, American English, perhaps a shortening of morphadike, a dialectal garbling of hermaphrodite; but bulldyker "engage in lesbian activities" is attested from 1921. According to "Dictionary of American Slang," a source from 1896 lists dyke as slang for "the vulva," and Farmer and Henley ("Slang and Its Analogues," 1893) has "hedge on the dyke" for "the female pubic hair."
[T]he word appears first in the long forms, bulldiker and bulldyking, both used in the 1920s by American blacks. No African antecedents have been found for the term, however, which leads to the possibility that this is basically just another backcountry, barnyard word, perhaps a combination of BULL and DICK. [Rawson]
