Many years ago, in the U.S. at least, the Restroom had a sort of foyer outside of the place where you "take care of business". there would be couches, tables, newspapers, etc. You actually could go in and sit down for a little while and relax. i think this is where the term comes from.
Again here in the U.S. the term is almost exclusivly used for a facility NOT in the home. in the home you have a bathroom (even if there is no bath there). at least, that's my experience.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors