Void mongo

A person who is so dumb that their mind seems to be an empty void until a random, usually nonsensical thought pops up which they can't help to share, only for their mind to return to it's usual state of complete and utter emptiness.

Matt: If I shave my head, would it be more or less weird to not shave my eyebrows as well?
Jenny: wtf?
Joe: don't mind Matt, he's a void mongo.