that'll be the day - definition in the British English Dictionary & Thesaurus - Cambridge Dictionaries Online (US)

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English definition of “that'll be the day”

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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!"
Translations of “that'll be the day”
in Chinese (Traditional) (表示不太相信)胡說,哪有這樣的事…
in Chinese (Simplified) (表示不太相信)胡扯,哪有那样的事…
(Definition of that'll be the day from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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