Short for "big fat whiny baby" (not referring to size) but a general random insult.
Somedays my gf can be a bfwb. Until I make sweet love to her, then she's a bfb.
Best Friend With Benefits.
An arrangement in which two people add a romance-free sexual component to an otherwise Platonic relationship. The parties involved may have once been a couple, but not necessarily.
Girl A: "I saw you at the restaurant with your man last night. From that smile on your face, it looks to me like you had a good time after dinner!"
Girl B: "Oh, he's not my man per se...he's my BFWB."
Girl A: "So...you mind if I ask him out?"
Girl B: "Be my guest!"