Discussion Forum—A Way with Words, a fun radio show and podcast about language

 From Wikipedia's entry for toilet:

The origin of the (chiefly British) term  loo  is unknown. According to the OED, the etymology is obscure, but it might derive from the word Waterloo. The first recorded entry is in fact from  James Joyce'sUlysses  (1922): "O yes, mon loup. How much cost? Waterloo. Watercloset".

Other theories are:

• That it derives from the term "gardyloo" (a corruption of the French phrase  gardez l'eau  (or maybe: Gare de l'eau!) loosely translated as "watch out for the water!") which was used in  medieval  times when  chamber pots  were emptied from a window onto the street. However the first recorded usage of "loo" comes long after this term became obsolete.
• That the word comes from  nautical  terminology, loo being an old-fashioned word for  lee. The standard nautical pronunciation (in  British English) of  leeward  is  looward. Early ships were not fitted with toilets but the crew would urinate over the side of the vessel. However it was important to use the leeward side. Using the  windward  side would result in the urine blown back on board: hence the phrases 'pissing into the wind' and 'spitting into the wind'. Even now most yachtsmen refer to  the loo  rather than  the  heads.
• That the word derives from the 17th century preacher  Louis Bourdaloue. Bourdaloue's sermons at the Saint Paul-Saint Louis Church in Paris lasted at least three hours and myth has it that wealthier ladies took along "travelling"  chamber pots  that could be hidden under their dresses whenever the need arose to avoid the need to leave. Due to the popularity of the myth the bowls became known as Bourdaloues after the preacher and the name became corrupted to portaloos and sometimes just plain loos due to the habit of shortening words in slang.^{[citation needed]}