means The spine is the column of linked bones running down your back that supports your body and protects the spinal cord — and, by extension, the central supporting part of anything, or the courage to stand firm.

from From Latin spina, meaning a thorn or prickle, which also came to mean the backbone — perhaps because the bony vertebrae jut out like a row of thorns along the back. The word entered English through Old French in the late Middle Ages, carrying both senses: the literal ridge of bone and the sharp little spike, which is why botanists still call a plant's thorns 'spines.' The figurative 'courage' meaning is a much later flourish, the idea that a person with backbone won't bend.