buy vs knock off

buy

verb
  • To bribe. 

  • To make a purchase or purchases, to treat (for a drink, meal or gift) 

  • To make a bluff, usually a large one. 

  • To be equivalent to in value. 

  • To obtain (something) in exchange for money or goods. 

  • To obtain, especially by some sacrifice. 

  • to accept as true; to believe 

noun
  • Something which is bought; a purchase. 

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 buy and knock off occurred in a corpus of books? (source: Google Ngram Viewer )