Hexenschuss
A case of sudden onset lumbago, characterized by sudden inability to move.

It literally means "a shot of a witch" (not the injection shot, but the gun kind of shot). Considering the sudden onset of this kind of lumbago, usually without any discernible cause, one can imagine why it was imagined to be the result of some witch cursing someone. Nowadays we have obviously a much better understanding of back pain and after one week of diligently doing my exercises I'm almost completely back to normal.
I'm pretty sure my utter lack of any kind of regular exercise is more to blame for this incident of back pain than some random witch.