背筋
せすじ spine; line of the backbone
Why it means this
背 (the back) + 筋 (the muscles, the line) — the muscles along the spine; 背筋を伸ばす, to straighten up; 背筋が凍る, to be chilled to the bone.
Breakdown
Each character links to its full entry — origin story, stroke order, and readings.