incentive vs kick up the backside

incentive

adj
  • Inciting; encouraging or moving; rousing to action; stimulating. 

  • Serving to kindle or set on fire. 

noun
  • Something that motivates, rouses, or encourages. 

  • A bonus or reward, often monetary, to work harder. 

kick up the backside

How often have the words incentive and kick up the backside occurred in a corpus of books? (source: Google Ngram Viewer )