What one would a call something that is said following a good joke, making the person who said it look like a retard and causing the room one of those awkward silences.
ie. a joke that follows another but isnt funny.
This term was first used to label the jokes told by Simon Green but its uses are now widespread.
Someone funny tells a joke.
Someone else: Talking about that matter, what about your mum last night!
Other people in that group: That's such a lame Simon Joke.