Someone who has characteristics of defficiency such as a brokenleftelbow, unability to turn hands with palms facing upwards, and thinks like a 6 year old.
Yeah that kid that just got bit in the face by a dog, that's jack campbell the fucking retard.
Someone who is really...really mentally disabled and who has serious difficulties with even the most basic of tasks, for example walking. A common trait of a retard is tripping over vacuums in art rooms.