An 'Intellitard' is one who has a stunning intelligence level, but a lack of judgment to use it. This person usually has waves of high intelligence, followed by lows of supposed retardation, in a sense that is totally unexpected to concerned parties.
My genius son occasionally lacks common sense. He is an intellitard!
A mentally handicapped person who can perform common tasks (flushing a toilet), but fails to execute semi-complex tasks (counting change, multitasking).
For some reason, the manager always puts Jimmy on the cash register, even though he can never count the change back to the customers. You would think he would realize he's an intellitard and limit him to filling the condiment station...