Given that the wife of an Earl is still a countess, I am not sure about the rude word theory as cunt used to be a lot less taboo. The streets used by prostitutes throughout many towns in England were called Gropecunt Lane until well into the 1500's.
That's a good point, and countess has a pretty early attestation. Thinking about French too, con is pretty close to comte, although I don't know the history, and con is not obscene like in English currently.
The Wikipedia page on "Earl"[1] has a quote with citation agreeing with the theory. I wonder if that book has any more evidence.
Prostitutes have been taboo for quite some time so by association, that street name would be too. Interesting that it would be allowed as a public road name perhaps but I assume still taboo.
