backbone vs bravity

backbone

noun
  • Courage, fortitude, or strength. 

  • 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. 

  • Any fundamental support, structure, or infrastructure. 

bravity

noun
  • Synonym of braveness 

How often have the words backbone and bravity occurred in a corpus of books? (source: Google Ngram Viewer )