In the time before time, when not every film or show was a remake and men were men, people called each other "retard" as a slander towards intelligence. Then political correctness set it and many moons passed. "Retard" was deemed mean and other terms appeared yonder:
Attorney At Law
Now when people wish to insult someone's intelligence, they have more terms at their disposal although "retard" is considered bad taste. Eventually, these terms will be replaced by something new because they will be associated with meanness. I expect the new term will be "retarded" because by then the generation will have forgotten its meanness.
More seriously, "retard" is a catch phrase for anyone with something less than normal mental capabilities. There are various specifics underneath that umbrella term, and while I think it is our moral duty to treat these fellow human beings with dignity, respect, and help, it is generally difficult to have them participate in intellectual discussions, such as online forums, because they may often blurt out strange comments or unintentionally filibuster. I have direct experience with teaching aspergers folks in a typical college classroom and the results ranged from awkward to incredibly disruptive.