protection money vs slush fund

protection money

noun
  • Money paid, especially at regular intervals, to criminals or to corrupt officials who threaten to cause harm to the payer or to his or her business if the money is not paid. 

slush fund

noun
  • Money stored for illegal or dishonest purposes. 

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

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