hardship vs knock on wood

hardship

verb
  • To treat (a person) badly; to subject to hardships. 

noun
  • Difficulty or trouble; hard times. 

  • A burden, a source of difficulty that could impose a barrier. 

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