knock on wood vs trouble

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. 

trouble

verb
  • In weaker sense: to bother or inconvenience. 

  • To mentally distress; to cause (someone) to be anxious or perplexed. 

  • To worry; to be anxious. 

  • To take pains to do something. 

noun
  • A difficulty, problem, condition, or action contributing to such a situation. 

  • A fault or interruption in a stratum. 

  • A malfunction. 

  • A violent occurrence or event. 

  • Efforts taken or expended, typically beyond the normal required. 

  • Liability to punishment; conflict with authority. 

  • Wife. Clipping of trouble and strife. 

  • A distressing or dangerous situation. 

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