apprehend vs knock on wood

apprehend

verb
  • To acknowledge the existence of (something); to recognize. 

  • To be or become aware of (something); to perceive. 

  • To have a conception of (something); to consider, to regard. 

  • To seize or take (something); to take hold of. 

  • To understand. 

  • To be of opinion, believe, or think; to suppose. 

  • To seize or take (a person) by legal process; to arrest. 

  • To be apprehensive; to fear. 

  • To anticipate (something, usually unpleasant); especially, to anticipate (something) with anxiety, dread, or fear; to dread, to fear. 

  • To take hold of (something) with understanding; to conceive (something) in the mind; to become cognizant of; to understand. 

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 apprehend and knock on wood occurred in a corpus of books? (source: Google Ngram Viewer )