What is "in joke"

Joke \Joke\, n. [L. jocus. Cf Jeopardy, Jocular, Juggler.]

1. Something said for the sake of exciting a laugh; something witty or sportive (commonly indicating more of hilarity or humor than jest); a jest; a witticism; as, to crack good-natured jokes.

   And gentle dullness ever loves a joke.
   --Pope.

   Or witty joke our airy senses moves To pleasant laughter.
   --Gay.

2. Something not said seriously, or not actually meant; something done in sport.

   Inclose whole downs in walls, 't is all a joke.
   --Pope.

   In joke, in jest; sportively; not meant seriously.

   Practical joke. See under Practical.