loo

There are several theories about the origin of the British term for the smallest room in the house "loo".

The most popular, is that is derived from the cry of "gardyloo" from the French "regardez l'eau" (watch out for the water') which was shouted by medieval servants as they emptied the chamber-pots out of the upstair windows into the street.
Another theory, and more likely, refers to the trade name 'Waterloo', which appeared prominently displayed on the iron tanks in many British outhouses during the early 20th century.

Here are some synonyms, take your pick:

Ladies, Gents, latrine, lavatory, loo, netty, place of easement, powder room, privy, s***-house, bog, smallest room, thunder-box, toilet, and the universal WC (short for water closet).

For the verb "go to the lavatory" we use these euphemisms:

explore the geography of the house
go to the bathroom
pay a visit
powder my nose
visit the smallest room
wash my hands
see a man about a dog
... and many others