mad as crackers
A Brit who’s so out of their mind they think they’re in a fancy suit and not a total mess.
My friend’s cousin is mad as crackers. He talks to the ceiling and wears a sock on his head.
My neighbor said his dad is mad as crackers. He tried to teach the mailman how to play chess.
My brother’s friend is mad as crackers. He thinks he’s a king and everyone else is just peasants.