Fuck buddy friend. Someone who you use purely for sex.
Eric: "So do you like Butters then?"
Kyle: "No, not at all. She's just an FBF."
Eric: "Woah, that's harsh, she seems really nice but I guess I would too. Chins up."
FBF - Fat Best Friend, the compulsory companion of the attractive boy/girl you are trying to pull. It is best in these situations to get a friend to take one for the team.
Also see Nottie.
"Aww man, take one for the team with her fbf, so i can move in."