ETYM back , n . + bone.
1. The bone running down the back of vertebrate animal; the spine.
2. A computer network that connects other computer networks.
3. (Informal) Fortitude; SYN. grit, guts, sand, gumption.

The series of vertebrae forming the axis of the skeleton and protecting the spinal cord; SYN. vertebral column, spine, backbone, back.
The axial skeleton of a vertebrate that consists of an articulated series of vertebrae which extend from the neck to the tail and protect the spinal cord — called also backbone.