Named for Joss Whedon, creator of Buffy the Vampire Slayer and Angel. Generally used by fanfiction writers when the shows chose to go in a direction that contradicted the story the fanfiction writer was currently working on. Jossing can mean that the show's writers contradicted earlier episodes, but more often just means that they filled in backstory differently than some fans did.
"I just wrote a story about Spike's past as a pickpocket, but it turns out he was a wimpy poet when he was human! I've been Jossed!"