looyoudaoicibaDictYouDict[loo 词源字典]
loo: [20] Loo presents one of the more celebrated puzzles of English etymology. Not the least of its problematical points is that there is no reliable evidence of its existence before the 1920s, whereas most of its suggested sources have a more dated air than that. Amongst them, the most widely touted is of course gardy loo!, a shout of warning (based on French gardez l’eau ‘beware of the water’) supposedly used when emptying chamber pots from upper-storey windows in the days before modern plumbing; but that is chronologically most unlikely.

Other possibilities are that it is short for Waterloo, which was a trade name for cast-iron lavatory cisterns in the early part of the 20th century (‘O yes, mon loup. How much cost? Waterloo. Watercloset’, James Joyce, Ulysses 1922), and that it comes from louvre, from the use of slatted screens for a makeshift lavatory. But perhaps the likeliest explanation is that it derives from French lieux d’aisances, literally ‘places of ease’, hence ‘lavatory’ (perhaps picked up by British servicemen in France during World War I).
[loo etymology, loo origin, 英语词源]
loo (n.1)youdaoicibaDictYouDict
"lavatory," 1940, but perhaps 1922, probably from French lieux d'aisances, "lavatory," literally "place of ease," picked up by British servicemen in France during World War I. Or possibly a pun on Waterloo, based on water closet.
loo (n.2)youdaoicibaDictYouDict
type of card game, 1670s, short for lanterloo (1660s), from French lanturelu, originally (1620s) the refrain of a popular comic song; according to French sources the refrain expresses a mocking refusal or an evasive answer and was formed on the older word for a type of song chorus, turelure; apparently a jingling reduplication of loure "bagpipe" (perhaps from Latin lura "bag, purse").
From its primary signification -- a kind of bagpipe inflated from the mouth -- the word 'loure' came to mean an old dance, in slower rhythm than the gigue, generally in 6-4 time. As this was danced to the nasal tones of the 'loure,' the term 'loure' was gradually applied to any passage meant to be played in the style of the old bagpipe airs. ["Grove's Dictionary of Music and Musicians," London, 1906]
The refrain sometimes is met in English as turra-lurra.