I can't stand "retard" or "retarded" being used incorrectly and most of the time, they are. I've rarely heard anyone use it for the actual meaning, "to slow down", other than the "ritard" in music. If you're gonna call someone stupid, I mean, go ahead and do it I guess, but by God, don't make yourself look foolish and ignorant in the process by using "retarded."
And if any of you are thinking, "oh but there's a condition called mental retardation so hur hur it does mean x," think again. It was renamed to "intellectual disorder." The more you know!