This is an acronym describing one's involvement at a party when there are girls around. It is normally used by one friend who is not in attendance at said party talking to another friend who is (via text). M.F.O.I. means Mingling, Fingering, or Otherwise Interacting (in reference to the people at the party (mingling), the girls at the party (fingering; could be used literally, but it does not need to be)), or otherwise interacting (etc...).
Guy 1 (not at party): Hey dude, hows the party? There any girls there?
Guy 2 (at party): It's alright, and they ain't bad.
Guy 1 (not satisfied with answer): You M.F.O.I.?
Guy 2: Nah, I'm just hanging with the guys, playin' some beer pong.
Guy 1: C'mon man, play the field.
The day after. In some way, shape, or form you were inebriated the night before and you left something out of place or what not in your room that could lead to you getting busted. A sober search in the frantic look around your room in the morning to make sure you didn't do something stupid, like leave a bong next to your bed.
Guy 1: Dude, this morning I realized I left my bong, grinder, and a bag of hash next to my bed when I passed out.
Guy 2: Lucky you did a sober search before you got caught.