Yep, just one word :). But it's not general Polish, it's fandom slang, so if you used that word with people who aren't familiar with fandom, they wouldn't know what you mean. Fandom slang has so many borrowings from English it would probably be easier to work with for you than for an average Polish speaker ;).

It's even less formal than "loo", but I can't think of anything better. Thank you!