"Loo" is so common and seemingly long-established that some would be surprised to learn that the OED can find no written trace of it before 1922 (in Ulysses). And what is its origin?
The invaluable Oxford Dictionaries Online (formerly Ask Oxford) has this prim comment:
There are several theories about the origin of this common term for a familiar article of sanitary furniture. The first, and most popular, is that it is derived from the cry of "gardyloo" (from the French regardez l'eau or "watch out for the water") which was shouted by medieval servants as they emptied the chamber-pots out of the upstairs windows into the street. This is historically problematic, since by the time the term "loo" is recorded, the expression "gardyloo" was long obsolete.The OED will have none of these, and simply says "origin obscure".
A second theory is that the word derives from a polite use of the French term le lieu (the place) as a euphemism. Unfortunately, documentary evidence to support this idea is lacking. A third theory, favoured by many, refers to the trade name "Waterloo", which appeared prominently displayed on the iron cisterns in many British outhouses during the early 20th century. This is more credible in terms of dates, but corroborating evidence is still frustratingly hard to find.
Various other picturesque theories also circulate, involving references to doors numbered "00" or people called "Looe".