(If that's what people mean when they say "Joss'd." Is it? I'm not really in the whole Jossverse fandom, I just keep jumping from one Whedon show to another...
I think "being Jossed" originally referred to writing a (speculative) fic, and have it rendered impossible by events of the show. (Like if you wrote a fic during the season 2-3 hiatus about Fred helping Angel to deal with Buffy's death and the two of them starting a romantic relationship.) Then it started to be used about fan speculation that didn't pan out and fanon that what contradicted as well.

I love Fred's parents! Must re-watch this show soon.