knock on wood vs nag

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. 

nag

verb
  • To repeatedly remind or complain to (someone) in an annoying way, often about insignificant or unnecessary matters. 

  • To bother with persistent thoughts or memories. 

  • Anyone would think that I nagged at you, Amanda! (From Amanda! by Robin Klein) 

  • a nagging pain in his left knee 

  • The notion that he forgot something nagged him the rest of the day. 

  • To bother or disturb persistently in any way. 

noun
  • Someone or something that nags. 

  • A persistent, bothersome thought or worry. 

  • An old, useless horse. 

  • A repeated complaint or reminder. 

  • A small horse; a pony. 

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