Slang for "retard" or one who is mentally challenged. It is a term commonly used on Instant Messenger and is said to have even originated there. Some in fact only use it when Online.
Dan: Hey Pat, I think you are a re-re! What do you think of that?
Pat: Well Dan, I think you are a fucking asshole because we are supposed to use that word Online only!
Dan: Well at least I like women!
Pat: Damn! You got me there...