A long, drawn-out joke with an elaborate backstory, misguided details, and unnessesary exposition. What seperates this from other jokes is the deadpan ending, usually marked by something ridiculously stupid, a groan-inducing pun, or simply no punchline at all.
Usually told to make the joke teller laugh at the audience's dissapointment and confusion. Most of the time, some of the audience members join in the laugher as an attempt not to look like "the guy who didn't get the joke".
The most common Unjokes are: The Tennis Ball Joke, The Friar Joke, The Clown Joke, The Shaggy Dog Joke
I dont feel like typing out one of the incredibly long jokes... so here is a short one.
So two polar bears are in a bathtub together. One says to the other, "Hey, can you pass the soap?" The other says, "That's not soap... That's a radio!!" (Laugh for effect. Others should join in)