knapsack vs slipcover

knapsack

noun
  • A case of canvas or leather, for carrying items on the back. 

  • A set of values from which a subset is chosen. 

verb
  • To go hiking while burdened with a knapsack, usually overnight or for longer. 

slipcover

noun
  • A fitted protective or decorative cover that may be slipped off and on a piece of upholstered furniture, usually made of cloth. 

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