A sudden, acute lower back pain (acute lumbago) that often occurs when lifting or twisting — commonly called a 'strained back' or 'acute lumbar sprain'.
箱を持ち上げた瞬間にぎっくり腰になってしまった。
The moment I lifted the box, I threw out my back.
ぎっくり腰になったら無理に動かさず、早めに医者に診てもらったほうがいい。
If you strain your back, don't force yourself to move; it's best to see a doctor early.
彼はぎっくり腰で数日間仕事を休んだ。
He took several days off work because of a strained back.