in joke
/ˈɪnˌdʒoʊk/
noun
- A joke that is understood only by a particular group of people who share a special knowledge or experience.
- The movie is full of in-jokes that only fans of the original book will understand.
- At the office, there was an in-joke about the broken printer that always made the team laugh.
- The two old friends shared an in-joke about their college days that no one else found funny.