Wiktionary
inside joke
n. (context idiomatic English) A joke that is understood or meant to be understood only by certain people who are in the know about the details
Usage examples of "inside joke".
Debbie and Kady said in unison, a chef's inside joke that made Jane groan as she pulled Debbie away.
Putting the Polaroid in his coffin had been Desie's idea, an inside joke.
At the working level, rivalry between the Bureau and the Service largely disappeared, except for the occasional inside joke or two.
Early in the movie, a rapist named (inside joke) Quesada is acquitted in court.