aplomb vs balls of steel

aplomb

noun
  • Self-confidence; poise; composure. 

  • The apparent elegance and precision exhibited by a confident, accomplished dancer. 

  • The perpendicular; perpendicularity. 

balls of steel

noun
  • Courage, audacity. 

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