1931, American English, perhaps a shortening of morphadike, dialectal garbling of hermaphrodite; but bulldyker "engage in lesbian activities" is attested from 1921, and a source from 1896 lists dyke as slang for "the vulva."
[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]
Old English dic "trench, ditch; an earthwork with a trench; moat," from Proto-Germanic *dik- (cf. Old Norse diki "ditch, fishpond," Old Frisian dik "mound, dam," Middle Dutch dijc "mound, dam, pool," Dutch dijk "dam," German Deich "embankment"), from PIE root *dheigw- "to pierce, fasten" (cf. Sanskrit dehi- "wall," Old Persian dida "wall, stronghold, fortress," Persian diz).
At first "an excavation," later (late 15c.) applied to the resulting earth mound; a sense development paralleled by cognate forms in many other languages. This is the northern variant of the word that in the south of England yielded ditch (n.).
: That woman lives with her dyke daughter and her dyke daughter-in-lawnoun
A lesbian, esp one who takes an aggressive role; bulldyke
[1930s+; origin uncertain and much debated; perhaps fr a shortening of morphodyke, dialectal and substandard pronunciation of ''hermaphrodite,'' perhaps influenced by dick, ''penis''; a source of 1896 lists dyke, ''the vulva'']