in-joke 
noun
A joke that is understood or appreciated by only a small group of people: the in-jokes are fun if you get them...
- It is also an extremely funny illustration of the national obsession with word-play, in-jokes and notably filthy double entendre.
- I can share personal memories and in-jokes with him, and compliment him on his fine choice of tattoo.
- Occasionally I misused the column to secretly share an in-joke with a friend, or girlfriend.