XD = The X is the same as >.< and the D is laughing/smiling/giggling/happy emotion, as in the emoticon . So basically, you're laughing while going crosseyed. Some of us actually experience this. *Cough*
Word contribution: informal speech
Jeffrey Dahmer, David Berkovitz, Albert DeSavo,
Ted Bundy, Albert Fish, John Wayne Gacy,
Edward Gein, Charles Manson, and Harvey Carignan.