loo

Uncertain, although usually derived in some way from Waterloo, the site of Wellington's 1815 victory over Napoleon, likely via a pun based on water closet. Other suggested derivations include corruptions of French l'eau (“water”), lieu (“place”), lieux d'aisances (“'places of convenience': a lavatory”), lieu à l'anglaise (“'English place': a British-style lavatory”), bordalou (“a diminutive chamber pot”) or gardez l'eau (“'mind the water'”), via Scots gardyloo, formerly used in Edinburgh while emptying chamber pots out of windows; the supposed use of "Room 100" as the lavatory in Continental hotels; a popularisation of lew, a regional corruption of lee (“downwind”), in reference to shepherds…

出典: Wiktionary「loo」（CC BY-SA 4.0）