free pass vs let-off

free pass

noun
  • An intentional walk. 

  • An exemption from normal processes. 

  • A document entitling the bearer to free transportation. 

let-off

noun
  • An escape from punishment. 

  • A device for letting off, releasing, or giving forth, as the warp from the cylinder of a loom. 

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