Public Toilets In Different Countries

The term lavatory, abbreviated in slang to lav, derives from the Latin lavātōrium, which in turn comes from Latin lavō ("I wash"). The word was used to refer to a vessel for washing, such as a sink/wash basin, and thus came to mean a room with such washing vessels, as for example in medieval monasteries, where the lavatorium was the monks' communal washing area. The toilets in monasteries however were not in the lavatorium but in the reredorter. Nevertheless the word was later associated with toilets and the meaning evolved into its current one, namely the polite and formal euphemism for a toilet and the room containing it. Lavatory is the common signage for toilets on commercial airlines around the world.
• Loo
The origin of the (chiefly British) term loo is unknown. According to the OED, the etymology is obscure, but it might derive from the word Waterloo. The first recorded entry is in fact from James Joyce's Ulysses (1922): "O yes, mon loup. How much cost? Waterloo. Watercloset".