bull-dyke (n.)

"male of a bovine animal," c. 1200, bule, from Old Norse boli "bull, male of the domestic bovine," perhaps also from an Old English *bula, both from Proto-Germanic *bullon- (source also of Middle Dutch bulle, Dutch bul, German Bulle), perhaps from a Germanic verbal stem meaning "to roar," which survives in some German dialects and perhaps in the first element of boulder (q.v.). The other possibility [Watkins] is that the Germanic word is from PIE root *bhel- (2) "to blow, swell."

An uncastrated male, reared for breeding, as opposed to a bullock or steer. Extended after 1610s to males of other large animals (elephant, alligator, whale, etc.). Stock market sense "one who seeks to cause a rise in the price of a stock" is from 1714 (compare bear (n.)). Meaning "policeman" attested by 1859. Bull-necked is from 1640s. Figurative phrase take the bull by the horns "boldly face or grapple with some danger or difficulty" first recorded 1711 (Swift). To be a bull in a china shop, figurative of careless and inappropriately destructive use of force, attested from 1812 and was the title of a popular humorous song in 1820s England.

"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]