incentive vs slush fund

incentive

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

  • Something that motivates, rouses, or encourages. 

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

  • Serving to kindle or set on fire. 

slush fund

noun
  • Money that has not been hypothecated or dedicated to a particular purpose, or which is used for a different purpose than originally intended. 

  • Money stored for illegal or dishonest purposes. 

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