grandfather vs let off

grandfather

verb
  • To be, or act as, a grandfather to. 

  • To retain discontinued laws or rules for (a thing, person or organization previously affected by them). 

noun
  • A father of someone's parent. 

  • A male forefather. 

  • The archived older version of a file that immediately preceded the father file. 

let off

verb
  • To forgive and not punish. 

  • To cause to explode or come out; to release. 

How often have the words grandfather and let off occurred in a corpus of books? (source: Google Ngram Viewer )