heh. well, that's what we allus called it when I was growin' up. I think my Mom still uses the term.

edit: ah ha! Bartleby says:

biffy

SYLLABICATION: bif·fy
VARIANT FORMS: also biff
NOUN: Inflected forms: pl. bif·fies also biffs
Upper Midwest 1. An outdoor toilet; an outhouse. 2. An indoor toilet.
ETYMOLOGY: Perhaps alteration of privy.

(bold emphasis mine)

formerly known as etaoin...