n00b biscuit
When someone's so clueless they might as well be a biscuit that got dropped on the floor.
He didn't know what a prime number was. He's a n00b biscuit.
She tried to do her hair and it looked like a dog did it. She's a n00b biscuit.
He tried to dance and it looked like he was having a seizure. He's a n00b biscuit.