Etymology

Noun

1. A person or thing's backbone; the series of bones collectively from one's (literal or figurative) head to tail or pelvis.
   • 1851, Herman Melville, Moby Dick, Chapter 80,

     If you attentively regard almost any quadruped's spine, you will be struck with the resemblance of its vertebrae to a strung necklace of dwarfed skulls.

2. A rigid, pointed surface protuberance or needle-like structure on an animal, shell, or plant.
   • 1871, Charles Darwin, Descent of Man, Chapter XII,

     The male, as Dr. Gunther informs me, has a cluster of stiff, straight spines, like those of a comb, on the sides of the tail

3. Courage or assertiveness.
   • 2001, Sydney I. Landau, Dictionaries: The Art and Craft of Lexicography, Cambridge University Press (ISBN 0-521-78512-X), page 409,

     Trademark Owners will nevertheless try to dictate how their marks are to be represented, but dictionary publishers with spine can resist such pressure.

4. The narrow, bound edge of a book.
   • Powells Book's, rare books basics,

     Because the spine is generally all you can see when a book is on the shelf, the spine displays the title and author of the book and is often ornately decorated.