You think he will pay you back? That'll be the frosty Friday.
that'll be the frosty Friday
idiomSynonym of that'll be the day (“said in reply to something that one believes will never happen”).
Origin
disputed originProbably a humorous elaboration of that’ll be the day, with frosty and Friday chosen for the alliteration; probably also implicitly evoking a cold day in Hell, referring to when Hell freezes over.
My brother promised to clean the entire garage by noon, but that'll be the frosty Friday.
When the politician claimed he would lower taxes without cutting any public services, the skeptical voters knew that'll be the frosty Friday.