hesitate vs knock on wood

hesitate

verb
  • To stop or pause respecting decision or action; to be in suspense or uncertainty as to a determination. 

  • To stammer; to falter in speaking. 

  • To utter with hesitation or to intimate by a reluctant manner. 

knock on wood

verb
  • To take a customary action to ward off some misfortune that is believed to be attracted by a presumptuous statement. 

intj
  • Hopefully; a self-directive to undertake the customary action to ward off bad luck. 

How often have the words hesitate and knock on wood occurred in a corpus of books? (source: Google Ngram Viewer )