I'm looking for an adjective which I could use for example in the following situation:

> 
> "Don't try to understand the joke Joanne said to George; it's a
>  in-joke between romantic partners."
> 
> 
> 

becomes:

> 
> "Don't try to understand the joke Joanne said to George; it's a [X
>  adjective] in-joke."
> 
> 
> 

Does this "X adjective" exist? Is it defined in a thesaurus?