that'll be the day
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]
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Words nearby that'll be the day
thatch palm, that'd, that does it, that is, that'll, that'll be the day, that makes two of us, that's, that's _____ for you, that's about the size of it., that's how the ball bounces
The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.