When the writers of a television show, movie, video game or other product include jokes that the audience will get based not on the show, but on outside knowledge. Actors meeting co-stars from previous series, saying famous lines, or put in familiar situations just to play to the base like an asexual form of fanservice.
An in-jokey shout out in a body of work. The type of thing that fans will catch, but everyone else will ignore. More obscure than a simple reference, more in-depth than an in-joke, but a unique kind of thing anyway.
The writers are, in effect, winking at their fans. Fankwink.
Wow, they really put "you wouldn't like me when I'm angry" in the new Hulk movie? That's some Grade-A fanwink right there.