The Collaborative International Dictionary of English v.0.48:

Backbone \Back"bone"\ (b[a^]k"b[=o]n`), n. [2d back, n. + bone.]
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   1. The column of bones in the back which sustains and gives
      firmness to the frame; the spine; the vertebral or spinal
      column.
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   2. Anything like, or serving the purpose of, a backbone.
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            The lofty mountains on the north side compose the
            granitic axis, or backbone of the country. --Darwin.
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            We have now come to the backbone of our subject.
                                                  --Earle.
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   3. Firmness; moral principle; steadfastness.
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            Shelley's thought never had any backbone. --Shairp.
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   To the backbone, through and through; thoroughly; entirely.
      "Staunch to the backbone." --Lord Lytton.
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