when hell freezes over

US /wɛn hɛl ˈfriːzɪz ˈoʊvər/

UK /wɛn hɛl ˈfriːzɪz ˈoʊvər/

never; not ever

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I'll apologize to him when hell freezes over.

•

She'll agree to that plan when hell freezes over.