backbone

Etymology

From Middle English bakbon, bakebon, bac-bon; equivalent to back + bone. Compare the semantically analogous English ridgebone.

noun

1. The series of vertebrae, separated by disks, that encloses and protects the spinal cord, and runs down the middle of the back in vertebrate animals.
2. (figurative) Any fundamental support, structure, or infrastructure.
   Before automobiles, railroads were a backbone of commerce.
   Undoubtedly it can be said that the humble 0-6-0 has been the backbone for general service, or general utility on British railways right from their earliest days, and is likely to remain so. 1945 November and December, H. C. Casserley, “Random Reflections on British Locomotive Types—1”, in Railway Magazine, page 320
   With little regular employment available in East Kent the backbone of the Kent Coast passenger traffic is therefore the commuters, the not inconsiderable numbers of people who travel each day to their work in Faversham, Sittingbourne, the Medway Towns and most of all, London. 1959 April, P. Ransome-Wallis, “The Southern in Trouble on the Kent Coast”, in Trains Illustrated, London: Ian Allan Publishing, →ISSN, →OCLC, page 212
3. (figurative) Courage, fortitude, or strength.
   He would make a good manager, if he had a little more backbone.