An injection expressing a state of disinterest or lack of caring. Often used in an urban context, this short phrase is uttered when one wants to appear as one does not care about a situation that has just occurred.
Derived from French, and loosely translates from French to English as "I don't give a f**k."
Jason: Je m'en fous. Aaron's a bastard.
Anne: You shouldn't swear in public. You never know who might be listening.
Jason: Je m'en fous.