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inside joke noun

  /ˈɪnsaɪd ˌd͡ʒoʊk/ , /ˈɪnsaɪd ˌd͡ʒəʊk/
  • (idiomatic) A joke that is understood or meant to be understood only by certain people who are aware of the details (an ingroup).
 Insider, Insiderwitz
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