Noun
inside joke (plural inside jokes)
(idiomatic) A joke that is understood or meant to be understood only by certain people who are aware of the details.
Synonyms: in-joke, private joke
After the incident with the potato peeler, references to potatoes became an inside joke among the witnesses.
Am I over existing or am I over existing? That's my inside joke. Ryan Trecartin
Piper: it looks like we have hole. Percy: Yeah we've got a dam hole! (LOL-ing) Piper: What! Percy: Inside joke. (still LOL-ing) Piper: Whatever. Rick Riordan