balls of steel vs spunk

balls of steel

noun

  • Courage, audacity. 

spunk

noun

  • Courage; spirit; mettle; determination. 

  • An attractive person (normally male). 

  • Touchwood; tinder. 

  • Semen. 

verb

  • To ejaculate. 

  • To waste (money etc.). 

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