Only the person who said it could really answer that question.
It's not an English expression.
But that's okay. We don't always express ourselves in standard, recognized, expressions. English would be boring and static if we didn't invent our own ways of saying things. We invent our own expressions.
There must be thousands of ways to insult a person's intelligence.
He's an airhead/waterhead/meathead/blockhead/chowderhead/knucklehead.
He's got nothing between his ears.
He's out to lunch.
He played football too long without a helmet.
And people will go on inventing more.
I hope you don't think I'm clueless.
It is a way of saying someone is a moron, brainless, or someone turned into a moron for a short time eg through lack of sleep, or a footballer who is slightly concussed after a hit on the head. It's like having 'marshmallow for brains'.