Cambridge Dictionaries online Cambridge Dictionaries online

The most popular online dictionary and thesaurus for learners of English

English definition of “that'll be the day”

that'll be the day

something you say in order to show you think that something is unlikely to happen: "Mike says he's going to give up smoking." "That'll be the day!"Impossible and improbable
(Definition of that'll be the day from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
Browse related topics

You are looking at an entry to do with Impossible and improbable, but you might be interested in these topics from the Chance and possibility topic area:

Add Cambridge dictionaries to your browser to your website

Translations of “that'll be the day”

Word of the Day

tankard

a large usually drinking cup, usually made of metal, with sloping sides, a handle, and sometimes a lid, mainly used for drinking beer

Word of the Day

Blog

Read our blog about how the English language behaves.

Learn More

New Words

Find words and meanings that have just started to be used in English, and let us know what you think of them.

Learn More