clef vs knock off

clef

noun
  • A symbol found on a musical staff that indicates the pitches represented by the lines and the spaces on the staff 

knock off

verb
  • To rob. 

  • To defeat. 

  • To halt one's work or other activity. 

  • To have sex with. 

  • To remove, as a discount or estimate. 

  • To make a copy of, as of a design. 

  • To assign (an item) to a bidder at an auction, indicated by knocking on the counter. 

  • To remove by hitting (something, someone) 

  • To kill. 

  • To accomplish hastily. 

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