Loo is a popular British colloquialism that is now Standard English for toilet or lavatory but its origin remains one of the mysteries of the English language. It is first cited from about the 1930s, which puts paid to theories that it derives from gardyloo, the cry emitted from chamber maids or housewives as they emptied buckets of dirty water (or worse) out of windows in the 17th century. Gardyloo is a corruption of the French guardez l’eau, which means ‘watch out for the water’. The problem is that gardyloo was long obsolete by the time loo meaning lavatory made its appearance. Another theory is British soldiers in WWI may have picked it up from the French lieux d’aisances, literally ‘places of comfort’ which is the euphemism used by the French for their toilets. To the average British ear, lieux would sound like loo but the theory remains dubious and unproven. Another theory is the trade name Waterloo, which commonly appeared on British sanitary ware, including toilets, from the early 20th century, but again, the case is unproven. The origin of loo remains unknown and is still a mystery.