Combination of "hijack" and "joke";
The surprise giving or receiving of a joke to or from a random stranger in a public or otherwise inappropriate setting.
Old Man: Excuse me, do you know what has 90 balls and fucks old ladies?
Roy: Um, no?
Old Man: BINGO!
(he runs away)
Roy: What was that?
Steve: We've just been the victims of a hijoking!