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.
When you go to a hot person's facebook page and begin to simulate what it would be like to fuck them.
Mark: Have you seen Taylor?
Robert: No, show me her Facebook.
Mark: This is what she looks like.
Robert: Oh man, I'm totally FBFing her tonight *moans*