horrendous vs unspeakable

horrendous

adj
  • Extremely bad; awful; terrible. 

unspeakable

adj
  • Extremely bad or objectionable. 

  • Incapable of being spoken or uttered 

  • Impossible to speak about. 

  • Unfit or not permitted to be spoken or described. 

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