During a TV show or book, when a character you've grown to love is inexplicably killed in the third act, usually as a quick way of evoking a great deal of emotion from the audience. Named for Joss Whedon, who literally has never produced anything in which this hasn't happened.
Penny is 'Whedoned' at the end of Dr. Horrible's Sing-Along Blog
Wash is 'Whedoned' at the end of Serenity
Buffy is 'Whedoned' in the season 5 finale of Buffy the Vampire Slayer
This literally goes on and on and on