knockback vs repulsion

knockback

noun
  • A rejection; a refusal. 

  • A blow that causes the recipient to fall or move backwards, a knock back; a recoil. 

  • Something that impedes or reverses progress; a setback. 

repulsion

noun
  • An extreme dislike of something, or hostility to something. 

  • The repulsive force acting between bodies of the same electric charge or magnetic polarity. 

  • The act of repelling or the condition of being repelled. 

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