Acronym for "Fuck Action -N- Train Action"; variation of the phrase "running the train" but F.A.N.T.A. describes a sexual experience involving at least 3 people; also known as orgy.
Bob: "So what's up with that girl you fucked last week? Does she have a friend for me to hook up with?"
Tom: "She said she did and that they're both down with the F.A.N.T.A.!!!"
Bob: "Well, fuck, dude, why haven't you called her? Lets get nasty!!!"