I’ve never heard “biff” for restroom, but “biffy,” yes.
Fascinating, isn’t it, how words can carry their own meaning so clearly in context?
OTOH, I once had a British boss who told me he was really “chuffed” about something. Onomatopoeia didn’t register for me: was “chuffed” something really really good or something really really bad?
So I asked him, and he looked at me like I was from Mars: “Chuffed! You know, just … chuffed!”
RBION didn’t mention that the woman in Washington had been feeding those raccoons for years.