What is "to the backbone"

Backbone \Back"bone"\ (b[a^]k"b[=o]n`), 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.

adv. through and through; thoroughly; entirely.