- A piece of humour that is understood only by a group of people.
- 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.
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.