ignore vs knuckle down

ignore

verb
  • To deliberately not listen or pay attention to. 

  • To pretend to not notice someone or something. 

knuckle down

verb
  • To get to work; to focus on a task. 

  • To submit; knuckle under. 

How often have the words ignore and knuckle down occurred in a corpus of books? (source: Google Ngram Viewer )