Variant of kthxbye and kthxbai, but with a mordant edge.
The original term kthxbye (okay, thanks, bye) is generally used to terminate a (n online) conversation in a quick manner, often with slight insult to the other party. The implication is that the other person is not even worth talking to.
kthxdie is like that, only more so, and with an added death wish.
Obi wan Kenobi: You can't win, Darth--if you strike me down, I shall become more powerful than you could possibly imagine.
Darth Vader: kthxdie.
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi