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that'll be the day



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Idioms and Phrases

That will never happen, that's very unlikely, as in You think I'll win the lottery? That'll be the day! Presumably this phrase is short for that will be the day worth waiting for , but it is nearly always used ironically, as in the example. [Mid-1900s]

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Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.

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