A Ree-Tard is somebody so stupid that the normal term 'retard' isn't enough.
This is a person who uses all caps when typing. Ignores punctuation, spelling and sentence structure when posting online.
Ree-Tard's are always off topic, haven't the slightest clue as to what they are doing, and when you point out that they are acting retarded they morph into Ree-Tard's and attack you with misspelled, poorly worded and overly mis -punctuated emails or posts.