A theory of where the word loo comes from, “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.” Via Ask Oxford.
From Ask Oxford, “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 upstair 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 numbered ’00′ or people called ‘Looe’.”
