Beloit Science Fiction & Fantasy Association. Someone who is a member of the aforementioned group, and as such socializes beyond the group very minimally, LARPs, plays Dungeons and Dragons, Boffs (the game), goes to Ball Room dances, participates in Rocky Horror Picture Show reenactment, and often wears medieval outfits.
Sig Chi - Have you ever seen that girl over there before?
Theta - Never. Judging by her weird outfit though, I'd say she's a BSFFA.