Origin of dyke = lesbian

I’m curious if anybody knows the etymology of the word “dyke” being used to refer to lesbians. I’ve heard the origin of queer and faggot, but never dyke. Can someone enlighten me? Someone once told me it was from a Celtic (possibly Welsh) term meaning “valley dweller”, but I’m skeptical.

Isn’t dyke a colloquialism for hermaphrodite?

From [=http://www.wordorigins.org/wordord.htm]here.

The O.E.D. says it’s obscure.