In-joke

It's an in-joke.

It's sort of a left-wing in-joke.

You wouldn't get it. It's an in-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.

Perhaps these quotations arose from some in-joke of Yule’s, but whatever the case the lexicographical worth of these citations is dubious.

Intentionally or not, the movie [Ralph Breaks the Internet] 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).