The idiom "be a cold day in hell" is used to express that something is very unlikely to happen or that it will never happen. It is a way of saying that the chances of something happening are almost impossible.
The phrase "cold day in hell" refers to the idea that Hell is a place of eternal fire and heat, so the idea of a cold day in Hell is completely opposite to what is expected. Therefore, if something is said to happen on a "cold day in hell," it means it is very unlikely to happen.
For example, if someone says, "I'll start liking broccoli when it's a cold day in hell," they mean that they dislike broccoli so much that it is impossible for them to like it. Another example is, "He'll apologize when it's a cold day in hell," which means that the person is so stubborn that they will never apologize.
In summary, the idiom "be a cold day in hell" is used to express that something is very unlikely to happen or that it will never happen.
• It would be a cold day in hell before my parents allow me to go out with friends on a school night.
• It would be a cold day in hell before I trust my ex-boyfriend again.
• It'll be a cold day in hell before my boss approves my leave request.
• It'll be a cold day in hell before the government passes a bill that benefits the common man.
• It will be a cold day in hell before I eat broccoli.
• It would be a cold day in hell before I accept an apology from someone who has repeatedly hurt me.