Then where does “biffy” come from? I live in Wisconsin and a number of transplanted Minnesotans and northern Wisconsinites refer to it as the biffy. And they have no idea why.
I believe there was (is?) a company that provides portable toilet service called Biffy such that the name appears on their portable toilets.
Sorry, no cite at hand, but there are several companies that provide such services that do have Biffy in their names.
I took a trip to England in 2004 and Ireland in 2007. I tried asking after the Loo several times in both countries, just to see. Absolutely no one was familiar with the term, be it a waiter, a bartender, a tour guide, etc. I rephrase it, “Where is the restroom/bathroom” and they point me right to it.