What is the meaning of the word "a cold day in hell"?

The phrase is often used for emphasis to express skepticism or disbelief regarding the occurrence of a certain event or action.

Definitions:

Is used as a prediction there is no chance some event or condition will ever happen. ADJECTIVE
- I'll be a cold day in hell when I sign over the lease of my land to you.
It is unlikely or impossible it will ever occur. ADJECTIVE
- It will be a cold day in hell before the two parties agree to anything.
time of occurrence of an event that will never happen noun
- It will be a cold day in hell before I take another math exam.
- It will day a cold day in hell whenever that happens, but it's a nice goal to have.