Definition of "until hell freezes over" in English

Prepositional phrase

forever; without success, no matter how long one perseveres.
- He can continue to be hard on himself until hell freezes over, because he's so self-centeredly pessimistic that there's nothing I can ever do to lighten him up.
- They can be bellyaching to me about their life's problems until hell freezes over, but it doesn't mean that they'll ever get my sympathy.
- I am prepared to wait for my answer until hell freezes over.