we had one of those when i was growing up near Pittsburgh, my dad worked in construction. I never heard that phrase for them though. cool.
What back door? You enter straight into the basement. I have a shower and a toilet in my basement. But not a bathroom.
I’ve seen them in Philly row homes. In a friend’s house, it was in the converted coal room.
Pittsburgh Showers were generally accompanied by Pittsburgh Toilets (a bowl sitting oddly out in the open in the middle of the basement near the shower).
My wife grew up in Michigan. When we bought our first house she thought this was the weirdest thing she’d ever seen.