A slur used against people with mental dissorders such as autism, tourettes, ADHD, etc. It is meant to degrade these people and call them dumb, stupid, or other words of the sort because of their disorder.
Person 1: Hey, did you see Lary.
Person 2: Yeah, I heard he has autism.
Person 1: Fucking retard.
Someone mentally deficient, no friends and overall just clapped
Retrard
Bet you didn't spell that right....
Retard.