Clubhouse joke

An in-joke, also known as an inside joke or a private joke, is a joke whose humour is understandable only to members of an ingroup, that is, people who are in a particular social group, occupation, or other community of shared interest. It is an joke that is humorous only to those who are aware of the circumstances behind it.

In-jokes may exist within a small social clique, such as a group of friends, or extend to an entire profession such as the film or professional wrestling industries, or a particular sporting field or chess team. Ethnic or religious groups usually have their own in-jokes.

In-jokes are cryptic allusions to shared common ground that act as triggers; only those who have shared the common ground provide an appropriate response. An in-joke works to build community, sometimes at the expense of outsiders. Part of the power of an in-joke is that its audience knows that there are those who do not understand the joke.

An in-joke can also be used as a subtext, where people in the know may find humour in something not explicitly spoken. They may even apologise for doing so to a rookie, directly or indirectly stating that what they were laughing at what was an in-joke.

In the computer industry some computer programmers hide in-jokes within the code of software in the form of Easter eggs, i.e., hidden content that can be revealed only by following a specific sequence of inputs. Easter eggs is a way for software developers to put their "signature" on the program, especially in video games, originally during times when they received little or no credit from their employer for their work.

The Jargon File is a glossary of hacker slang, much of which is in-jokes or is based on in-jokes.

...
Wikipedia