in-joke vs private joke

in-joke

noun

  • Synonym of inside joke (“a joke that is understood or meant to be understood only by certain people who are aware of the details”) 

private joke

noun

  • Synonym of inside joke (“a joke that is understood or meant to be understood only by certain people who are aware of the details”) 

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