A Root
A person who acts like they’re stupid on purpose, even though they’re not. It’s like they’re pretending to be a brain-dead goldfish just to annoy you.
"My cousin is a root. He says ‘What’s a brain?’ every time I talk to him."
"At school, the root said ‘What’s a question?’ when I asked him a question."
"That root said ‘What’s a day?’ when I told him to do his homework on Monday."