"when hell freezes over" の定義と意味

What is the origin of the idiom "when hell freezes over" and when to use it?

The phrase "when hell freezes over" is an idiom used to convey the idea that something is so unlikely or impossible that it will never occur. The expression likely stems from the perception that hell, being a place of extreme heat and torment, could only freeze over in the most unimaginable and improbable circumstances.