dig,ditchと同じ語源を持つdikeのスペル変化から。 lesbianismの語源は不明だが、おそらく女性のマンコを意味するdykeのスラングから。
[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]