"Joss" is indeed an unusual name! Mr. Whedon is not the only person with it, but he is one of only a few. :-)

The fan-term "to be jossed" originally emerged in direct response to how Buffy the Vampire Slayer unfolded across seasons, often setting up questions at the end of one season, going on summer break leaving fans wondering and theorizing, and then coming back and resolving the questions in ways that contradicted the fans' imaginings.

(Above, Ysabetwordsmith shared an observation that I imagine suggests that the term may have subsequently picked up Firefly connotations, which of course weren't there in the original reference.)