Attempting to make a joke before all the key parts that are necessary to make the joke funny are there; often with the hope that the missing parts will form by the time they are required.
Person 1: That was uncool.
Person 2: What's uncool is your... ... um... damn, premature ejokulation.
Steve: Did you like The Simpsons Movie?
Matt: Ah, it was alright but the funniest thing was Spider-Pig and they used it in the trailer.
Steve: Yeah, I know what you mean, I hate premature ejokeulation.