backbone vs brass ovaries

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. 

brass ovaries

noun
  • courage, guts 

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