n. toilet. (Originally and primarily British.) : I gotta use the loo. Be with you in a minute.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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Word Origin & History
loo (1)
"lavatory," 1940, but perhaps 1922, probably from Fr. lieux d'aisances, "lavatory," lit. "place of ease," picked up by British servicemen in France during World War I. Or possibly a pun on Waterloo, based on water closet.