In
fandom, to have a previously held theory or idea, particularly in a
fanfiction story made impossible because of an unexpected development in
canon. Being Jossed only happens when the source canon is still open, as in the case of a television show still being in production.
The word was coined in Buffy fandom, where it was common for a fanfic that was canonical when written to lose its canonicity due to developments only weeks later. This was blamed on the (generally approved)attitude of series creator Joss Whedon that the characters and situations on the show should change freqently rather than remaining static, as is more common in television; so the experience of becoming incompatible with canon was named in his honor.
A story set in the future may be Jossed when a character who appears in the story is killed in canon. Fanfic is often Jossed when new facts are revealed about a character's past or personal tastes. When a story is no longer possible within canon, it becomes AU -- a story set in an alternate universe. Being Jossed is also sometimes referred to as "being AU'ed," especially in fandoms that existed before Buffy. "Jossed," however, is increasingly the standard term across fandoms, possibly because of the apropos similarity to "jostled." When a former element of canon itself is later made noncanonical, either through carelessness in continuity or deliberate retcon, this is usually not called being "Jossed," except humorously1.
Theories and ideas outside of any particular fanfic, such as the identity of a murderer or the predestination of a particular couple, can also be said to be Jossed, but the word is generally only used of ideas that have been in discussion in the fandom for some time.
1. e.g. "The Star Trek writers are so careless one of these days they're going to Joss themselves."