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Why do they call it the loo?

(It’s going to be hard to write this one without resorting to all sorts of unclever puns, but I’m going to do my best.)

When I was in London a couple of weeks ago, a group of us was sitting around in a pub on Saturday afternoon (what a cliché!) and someone mentioned that the reason that the English “loo” is so named because the toilet was commonly located in room 100 of buildings and the two (“loo” and “100”) look very much the same. (You can see that I jotted that tidbit down on my analog Palm Pilot (upper right quadrant) for later reference.) Turns out that pub chat aside, the jury is somewhat out on the etymology of “loo” (unless the OED, which I don’t have access to, says otherwise tons of people wrote in with the OED entry for loo, summarized below).

One popular theory comes from this timeline of toilets:

When people flung their potty waste out of the window, they would shout “Gardez l’eau” [gar-day low]. That’s French for “watch out for the water”. We probably get the word “loo” from this expression, although some people think it comes from “Room 100” which is what European people used to call the bathroom.

Wikipedia backs this version as well (don’t miss the list of euphemisms for toilet there, including poop-house (wtf?), dunny, and necessary).

Michael Quinion offers a few more theories. The word appears to originate no earlier than James Joyce’s usage in Ulysses in 1922 — “O yes, mon loup. How much cost? Waterloo. water closet.” — perhaps Joyce came up with it. Or it could be “a British mispronunciation of the French le lieu, “the place”, a euphemism.” Maybe loo is short for bordalou, “a portable commode carried by eighteenth century ladies in their muffs” (!!). Quinion also notes that “a rather more plausible [theory] has it that it comes from the French lieux d’aisances, literally ‘places of ease’ (the French term is usually plural), once also an English euphemism, which could have been picked up by British servicemen in World War One” but that there’s no real conclusive evidence to support any of these theories over the others.

Cecil Adams of Straight Dope offers many of the same theories as well as this additional one:

It’s short for “Lady Louisa,” Louisa being the unpopular wife of a 19th-century earl of Lichfield. In 1867 while the couple was visiting friends, two young wiseacres took the namecard off her bedroom door and stuck it on the door of the bathroom. The other guests thereafter began jocularly speaking of “going to Lady Louisa.” In shortened form this eventually spread to the masses.

But Adams has no definitive answer either and so the question of the etymology of loo will continue to be debated on the Internet and in pubs around the world.

Update: the OED notes Joyce’s usage as the earliest, but is also at a loss to explain things:

A. S. C. Ross’s examination of possible sources in Blackw. Mag. (1974) Oct. 309-16 is inconclusive: he favours derivation, in some manner that cannot be demonstrated, from Waterloo.