A person who is unusually short, slow, and adorable for one's age. A male must be under 5'5" when a Sophomore or later and a Female must be under 5'1" and not be able to run a 5.7 40 yard dash. Also it could be put to an extreme if you are a male and you sit on your girlfriend's lap instead of her sitting on yours.
Senior 1: Did you see that baby roaming the halls of Elder?
Senior 2: Yeah, what a Freshman!
Sophomore: No, he's actually in my grade, and my homeroom.
Senior 2: Wow, What a Slim Dith!