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A fanwork is jossed when new canon invalidates the story's character backstory, plot, etc. Jossing is a common occurrence with fanfic written for an open canon. The term originally comes from the Buffy the Vampire Slayer and Angel fandoms (both shows created by Joss Whedon), but has spread beyond the Jossverse.
For instance, in Stargate Atlantis fandom, someone who wrote a story when the first season was airing about how Atlantis never re-connected with Earth would have been jossed when contact was re-established in the second season.
Star Wars is a fandom that was constantly jossed.
A very early awareness of jossing is in Spockanalia #1 (1967): "Yeds are aware that when the new season begins, two weeks after our anticipated publication date, many (all?) of our lovely theories will be blown to pieces. On the other hand, we console ourselves with thoughts of all the new material we'll have to play with."
In 2015 a fan on twitter mused on how fandom used to be: "getting jossed was a huge blow that completely invalidated your fic. Now the kiddies don't even have a term for it." 
- For example, this post at Word Origins, dated July 1, 2002, says that the term was named "after the creator of Buffy the Vampire Slayer Joss Whedon, who is famous for unexpected plot and character twists".