Meaning of “that'll be the day” in the English Dictionary

british dictionary

"that'll be the day" in British English

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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!"

(Definition of “that'll be the day” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)