castoff vs knockback

castoff

noun
  • Something that has been rejected or discarded; a reject. 

adj
  • discarded, rejected. 

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. 

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