"loo"

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This British colloquial word for "toilet" was established usage
by the 1920s.  Suggested origins include:
French lieu d'aisance = "place of easement"
French On est prie de laisser ce lieu aussi propre qu'on le trouve
    = "Please leave this place as clean as you find it"
French Gardez l'eau! = "Mind the water!" (supposedly said in the
    days before modern plumbing, when emptying chamber pots
    from upper-storey windows.  According to Chris Malcolm
    (cam@aifh.ed.ac.uk), this phrase is still sometimes used by
    common folk in Edinburgh when heaving water or slops, and
    tour guides say that it originated there circa 1600.)
"louvre" (from the use of slatted screens for a makeshift lavatory)
"bordalou" (an 18th-century ladies' travelling convenience)
"looward" or "leeward" (the sheltered side of a boat)
"lee", a shepherd's shelter made of hurdles
"lieu", as in "time off in lieu", i.e., in place of work done
"lavatory", spoken mincingly
"Lady Louisa Anson" (a 19th-century English noblewoman whose sons
    took her name-card from her bedroom door and put it on
    the guest lavatory)
a misreading of room number "100" (supposedly a common European
    toilet location)
a "water closet"/"Waterloo" joke.  (James Joyce's Ulysses (1922)
    contains the following text:  "O yes, mon loup.  How much
    cost?  Waterloo.  water closet.")