As a verb, it means to dump a bunch of exposition at the end of a movie or TV show that has radical consequences for the story (e.g., the exposition states that the main character later died tragically), especially if it would have been better to show this development instead of describe it.
The term comes from episode 4F12 of the American animated series "The Simpsons" ("The Itchy & Scratchy & Poochie Show"). When the new addition to the animated show-within-the-show "Itchy & Scratchy", Poochie (voiced by Homer), proves extremely unpopular, the producers decide to kill him off. Homer tries to persuade them otherwise by reading a touching speech as Poochie that he thinks will end up in the show and endear the viewers to him. Instead, in the final cut, somebody else provides Poochie's voice, and states that the character has to return to his home planet. Crude animation then depicts Poochie "beaming up", and a note is tacked on that reads, "Poochie died on the way back to his home planet."
Real-life example, from Fark.com (discussing the show "Quantum Leap"):
Farker: "NBC canned it before it could be given a proper funeral. They Poochied the ending."