The 'tardle shell' is the turtleshell-like tiny backpack so frequently seen on the morbidly obese and mentally challenged folks in our society.
You often see heavy set people with these backpacks, which have two straps (usually black and/or thin, corded). These straps go around the shoulders to hold the tardle shell in place in the middle of the back.
The name 'tardle shell' comes from 'turtle shell' and 'retard', since often those called 'retards' wear these backpacks like shells.
Did you see that fat chick crossing the street? She had on a tardle shell!