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that'll be the day

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

BEHIND THE PHRASE

What does that’ll be the day mean?

That’ll be the day is a phrase that means something is never going to happen or is incredibly unlikely to happen, as in That’ll be the day I apologize for standing up for my sister! 

That’ll be the day is most often used when a person is being ironic, sarcastic, or skeptical. When used in that way, the phrase is often said with contempt or smugness.

That’ll be the day is used in the same way as phrases like “when pigs fly,” “fat chance,” “not in a million years” and many others that express extreme doubt or skepticism that something will actually happen.

Example: You think I am going to win the contest? That’ll be the day! I never win anything.

Where does that’ll be the day come from?

The first records of that’ll be the day come from around 1916. Originally used in Australia and New Zealand, the phrase was first used to mean “that will be a day worth waiting for.” Today the phrase is nearly always used ironically to imply that the day is never going to come.

You can use that’ll be the day when you think something is never going to happen, like all ice cream becoming free. When spoken, that’ll be the day is often accompanied with a sound or facial expression, such as a snort or smirk, that further emphasizes a person’s skepticism.

Did you know … ?

What are some synonyms for that’ll be the day?

What are some words that share a root or word element with that’ll be the day

What are some words that often get used in discussing that’ll be the day?

How is that’ll be the day used in real life?

That’ll be the day is a phrase used to mean the speaker thinks that something is never going to happen.

Try using that’ll be the day!

True or False?

That’ll be the day means a person is waiting for something they think is going to happen fairly soon.

How to use that'll be the day in a sentence

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