Re: Beware labeling others...lest you be labeled! ;-)
I think I follow your insult, but it took some thought to puzzle it out.
The problem is that it's hard to see how the noun form of retard could apply to a person. What you probably meant to say is retarder, "one who retards", but that doesn't quite have the ring you were looking for, does it?
My sympathies on the belly flop.
--Robert