Source : Webster's Revised Unabridged Dictionary (1913)

Backbone \Back"bone"\, n. [2d back,n.+ bone. ]
   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. --Darwin.

            We have now come to the backbone of our subject.
                                                  --Earle.

   3. Firmness; moral principle; steadfastness.

            Shelley's thought never had any backbone. --Shairp.

   {To the backbone}, through and through; thoroughly; entirely.
      ``Staunch to the backbone.'' --Lord Lytton.