knock on wood vs scruple

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. 

scruple

verb
  • To hesitate or be reluctant to act due to considerations of conscience or expedience. 

  • To excite scruples in; to cause to scruple. 

  • To regard with suspicion; to question. 

noun
  • Hesitation to act from the difficulty of determining what is right or expedient; doubt, hesitation or unwillingness due to motives of conscience. 

  • A weight of ¹⁄₂₈₈ of a pound, that is, twenty grains or one third of a dram, about 1.3 grams (symbol: ℈). 

  • A Hebrew unit of time equal to ¹⁄₁₀₈₀ hour. 

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