In LJ's 6/24 blog entry thing about Mexico (day 2) she mentioned the "biff" in the house they are staying at. The use of the word baffled most everyone here--I'm vaguely familiar with it, having hear "biffy" in reference to an outhouse before. However, LJ seems to call the entire bathroom as "biff" (and when accompanied with her slang of "flusher" and "douche," it's just terrible). However, I was recently going through the archives trying to find a strip, when I came across this one from Mike's time in Tokyo
wherein he calls the TOILET (not an outhouse!) a biff! So apparently this isn't a slang word LJ made up on the spot, but she's been using it for awhile (although in bizarre contexts).