expression, godan verb (-ru)
feel a chill run down one's spine
Idiomatic expression describing a sudden shiver of fear, dread, or unease, as if a cold sensation travels along the back.
その話を聞いて、背筋が寒くなった。
Hearing that story, I felt a chill run down my spine.
Similar meaning but stronger, often used for extreme fear or shock; literally 'one's spine freezes'.
Compound of 背筋 (spine) + が (subject marker) + 寒くなる (become cold). The phrase is a natural metaphorical extension of the physical sensation of cold along the back when frightened.