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that'll be the day

Idioms  

  1. 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]

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

“I get up every day and I live in hope that that’ll be the day that I’ll be surprised by the news,” began Stephen Colbert.

From The Guardian • Jun. 26, 2018

It’s very exhausting to wake up every single day and be afraid that’ll be the day that you start to lose the baby.

From Forbes • Aug. 2, 2015

If peple behaved in a civil manner when on foot, on bikes or in cars – well, that’ll be the day.

From New York Times • Apr. 23, 2010

Well, in the words of old Buddy Holiday, that'll be the day.

From Time Magazine Archive

Yeah," the boy grumbled, "that'll be the day.

From Frigid Fracas by Reynolds, Mack

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.