I've been doing research on LGBTQ+ terminology recently and I've come across pretty much the same sentence about the origin of the English derogatory/reclaimed term "dyke":
a source from 1896 lists dyke as slang for "the vulva."
However, the OED's first record referring to lesbian women isn't until 1931. I thought that perhaps the 1896 date on the Online Etymology Dictionary originated from the same source as this passage:
[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]
Which I was able to find in Rawson's 1989 Wicked Words... But not the mysterious 1896 source...
Was this 1896 source made up? Does it actually exist? Any clues to its whereabouts would be much appreciated!
Thanks so much!
EDIT: I'd like to sincerely apologize for the above language, specifically "Was this 1896 source made up?" After conversing with Mr. Harper via email, as Laurel suggested in the comments, he has been extremely helpful and I do not want anything to reflect negatively on his credibility or the credibility of his site, which is a fantastic resource for etymology. I was under the impression that sources had cited other sources in a sort of phone-tag manner, where none were actually the 1896 source. Mr. Harper has clarified that this is not the case.
EDIT 2: Turns out my university library has access to the non-condensed version of Green's Slang Dictionary, which puts a specific reference as "c. 1930 (ref. to late 19C) N. Kimball Amer. Madam (1981) 19: I had a housekeeper - usually an old dyke." The term 'bull-dyke' is noted even earlier: "1892 Decatur (IL) Daily Rev. 29 July 7(?)/2: ... She went to Blanche Alexander's place, at 101 Custom House place, in search of Belle Watkins, who, she said, had won the affections of Harvey Neal, alias 'Bulldyke.' Bell [sic] heard of her coming and escaped, but as soon as the woman got inside of the house she began firing right and left." However, it still seems like the 1896 source isn't in Green's... (unless the late 19C reference is to the 1896 source).