Word History: The story goes that a medieval French king was drenched with the contents of a chamber pot one day in Paris. The result was a rule that all Parisians exclaim, "garde à l'eau!" (look out for the water!) before dumping liquid waste out the window. Because of the "Auld Alliance" between Scotland and France against the English, the term eventually migrated to Edinburgh, where it was shouted by inhabitants of the tall medieval tenements in that city's Old Town before the contents of chamber pots were flung out onto the streets below.
The term "loo" is derived from Gardyloo.
This is probably not something you want associated with doing in your pants...