Pronounced like "foof," F.O.O.F is a faster way of saying, "Fresh out of Fucks." This statement is used when attempting to convey just how little regard one might have toward a particular person or situation.
Joe: "Hey man, I hear your ex is talking shit about you."
Mike: "Gimmie a second." *Digs around in pockets* "Sorry dude, I'm F.O.O.F."