That’ll be the day

What is That’ll be the day?

That’ll be the day definition and meaning on Dictionary terms:
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]

 

reference: www.dictionary.com/browse/that-ll-be-the-day

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