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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 Grammar:Nowadays, these days or today?We can use nowadays, these days or today as adverbs meaning ‘at the present time, in comparison with the past’:
(Definition of that'll be the day from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)

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