Someone who is perfectly capable of accomplishing a task but fucks it up anyway.
John, you put the roof shingles on backwards. Stop being a retard.
Smart Person: You're retarded.
14 Y/o white girl: OMG OMG OPMG ABLEIST
A retard is somebody who is fucking dumb as fuck such as Newton since he a mad fucking retard
Son: Why is my mate so damn retarded.
Dad: Probably because your a fucking spazzy retard yourself you bum fucking prick.
Person1: who's Rylan
Person2: a fucking retard