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 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.
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 number 00 or people called Looe.