What does it mean when a person says "I knocked out my back?"What happened to his back?
same as the phrase 'threw out my back' It means something is not in alignment or a muscle has been badly pulled. Most folks who say this cannot stand up straight and are hunched over until it is fixed- which can take a while
I'm guessing he knocked his vertebrae out of proper alignment. That happens to me on occasion and if it goes too far out of alignment it can be excruciating.
Threw out there back?
It means his back hurts. It is one of those vague terms used by laymen to describe any number of possible conditions.