Green’s Dictionary of Slang

1. (US) a privy, an outdoor lavatory.

1950		WELS.
1965–70		in DARE.

2. (Can./US) an indoor lavatory.

1942		Berrey & Van den Bark Amer. Thes. Sl.
1953		S. Bellow Augie March (1996) 303: I had to go to the biffy to take a leak.
1960, 1975		Wentworth & Flexner DAS.
1967–8		Baker et al. CUSS 79: Biff [...] Biffy Washroom.
1970		Meyer & Ebert Beyond Valley of the Dolls [film script] Hey, I've heard about trees growing in Brooklyn, but ferns in the biffy?
1997–2002		Alt. Eng. Dict. 🌐 biffy (noun, count, definite) bathroom, restroom.

3. (Can./US campus) a portable lavatory.

1989

P. Munro Sl. U.