frag vs knock off

frag

verb
  • To kill. 

  • To hit with the explosion of a fragmentation grenade. 

  • To deliberately kill (one's superior officer) with a fragmentation grenade. 

  • I fragged him but he fell off the ledge afterwards. 

noun
  • A successful kill in a deathmatch game. 

  • A fragmentation grenade. 

knock off

verb
  • To kill. 

  • 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 accomplish hastily. 

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