Nuttier than a Squirrel's lunch
Anyone who is remarkably bat-shit crazy, nuts or waaaayyyy beyond the eccentric
Tim: "Ugh, I have to spend the weekend with crazy grandpa Eugene."
John: "Oh, that one."
Tim: "Yeah, he's nuttier than a Squirrel's lunch."
John: "Oh, that one."
Tim: "Yeah, he's nuttier than a Squirrel's lunch."