"Crunch Time"

A Crash Course in Classroom Jargon:Crunch time: Part of a game where the result is still yet to be determined, characterized by extreme pressure. In business, a time when each decision is critical to the final success or failure of an operation.

I never would have guessed this came from sports. I would have assumed it came from the feeling that you were about to be crushed, or "crunch" as in "sit-up" because they're painful and not fun.