in-joke

• IPA^(key): /ˈɪnˌd͡ʒəʊk/

in-joke (plural in-jokes)

1. Short for inside joke.

   Well, it's a bit of an in-joke, but Fred has been going to start that diet "tomorrow" for the last few months.

   We have an in-joke; whenever someone sings a wrong note, they wipe the mistake off their music with an obvious gesture.

   • 14 November 2018, Jesse Hassenger, AV Club Disney goes viral with an ambitious, overstuffed Wreck-It Ralph sequel

     Intentionally or not, the movie makes Disney feel as enormous as the internet itself, containing a series of micro-targeted idiosyncrasies and in-jokes that are nonetheless controlled by a cultural monolith (whether that’s Disney or whatever massive corporation owns your local ISP).

An in-joke can carry the connotation of being slightly sinister, particularly if the subject of the humour is unaware that people find it funny. However in situations where everyone involved is in the in group, they are harmless - if slightly confusing to anyone not party to the joke.

• inside joke
• private joke