ex dee is sometimes used in place of XD, which is used for expressing laughing so hard you're squinting. Ex dee is (not so) commonly used in instant messaging to try to be funny.
I saw you practicing kissing in the mirror exdee!!!!
A fanfiction-type thing started by Trigger featuring members of Sonic Classic, Sonic CulT, The Moogle Cavern, and STH Area 51. Narration is assisted with screenshots of games that match the events.
<Ryoga> Kurai, you can sit on me, but at least take your skirt off first.
<CulT> NOW ISN'T THE TIME RYO
<Ryoga> ;_;
--excerpt from Ex Dee 3
Ay bee see dee ee eff jee aitch eye jay kay elle emm enn oh pee queue are ess tea you vee dubeu ex why zed. Now I know my ABC’s. Next time won’t you sing with me?