Backbone /(băk"bōn`)/

Back·bone

Backbone

n.
  1. The column of bones in the back which sustains and gives firmness to the frame; the spine; the vertebral or spinal column.
  2. Anything like , or serving the purpose of, a backbone.
    The lofty mountains on the north side compose the granitic axis, or backbone of the country.
    We have now come to the backbone of our subject.
    — Earle.
  3. Firmness; moral principle; steadfastness.
    Shelley's thought never had any backbone.
    — Shairp.

Phrases & Compounds
To the backbone
through and through; thoroughly; entirely.