knock off vs trephine

knock off

verb
  • To remove by hitting (something, someone) 

  • 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 kill. 

  • To accomplish hastily. 

trephine

verb
  • To perforate with a trephine. 

  • To use a trephine during surgery. 

noun
  • A surgical instrument with a cylindrical blade used to remove a circular section of tissue, f.e. bone or cornea; a trepan. 

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