There's a story about how Fion met one of the many wives he had, Sadhbh, when he was out hunting. She'd been turned into a deer by a druid, but he was able to figure out that she was really human and eventually she was able to turn back into one, and he got her pregnant before that same druid turned her back into a deer and she disappeared. Fion searched for her for years, and eventually he reunited with his and Sadhbh's son Oisin, whose name literally means "young deer".

So yeah, I guess the whole "deerfucker" thing makes some kind of sense, but she wasn't even really a deer nor was she in the form of one when Fion got her pregnant, so it's still a pretty stupid joke.