Mad as a spoon

Posted by ESC on May 29, 2009 at 13:28

"Lizzie of Hilltop Farm.She's mad as a spoon, of course - believes in banshees and werewolves and such - but she's very reliable when it comes to berries and nuts." "Aunt Dimity: Vampire Hunter," by Nancy Atherton (2008, Penguin Group, New York) Page 111. It's a cozy mystery set in England.

Can anyone explain "mad as a spoon"?