expression, noun
sudden severe lower back pain; strained back
Colloquial, often humorous idiom for acute lower back pain that strikes suddenly, as if struck by a witch. Compare ぎっくり腰, which is the more clinical term.
See also: ぎっくり腰
朝起きたら魔女の一撃で動けなくなった。
When I woke up, I was hit by a sudden back pain and couldn't move.
くしゃみをした瞬間、魔女の一撃が走った。
The moment I sneezed, a sharp pain shot through my lower back.
The standard medical/clinical term for acute lower back strain. 魔女の一撃 is a more colorful, colloquial synonym.
Literally 'a witch's strike'. The expression likens the sudden onset of severe lower back pain to being struck by a witch. The exact origin is uncertain, but it is a modern colloquial coinage.