The word 'loo' is a colloquial British term, and sometimes Australian English, for a toilet or restroom.
It's generally considered a more informal and polite way to refer to the lavatory than some other slang terms.
The origin of the word is somewhat debated, with several theories including a French connection ('lieu' which meant 'place' and was connected with outhouses) or a Napoleonic war observation from the cry 'gardez l'eau' meaning 'watch out for the water' as the content of chamber pots was thrown from the windows.
Whatever its origin, the word has persisted as a common, casual term used in everyday conversation. .
WC
LAV
JOHN
PRIVY
TOILET CLOSET
LAVATORY
WATERCLOSET ENDLESSLOOP