"Je m'en fou," literally, "I fuck myself over it", is used to express "I don't care," or "I don't give a flying fuck." Despite its apparent offensiveness to English-speakers, it is a very mild phrase that could be found in polite conversation.
Also worth noting is the semantic difference between "Va te faire foutre," and "Va te faire enculer." "Va te faire foutre" means "Go fuck yourself (up the vagina)," while "Va te faire enculer" means "Go fuck yourself (up the anus)." Of course, both of these are anatomically impossible.