crazy as a loon

crazy as a loon

Insane. Often used jocularly. Don't leave me alone with Uncle Stu, he's crazy as a loon!
See also: crazy, loon
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

crazy as a loon

AMERICAN, INFORMAL
If you describe someone as crazy as a loon, you mean that their behaviour is very strange or silly. She was obviously crazy as a loon.
See also: crazy, loon
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

crazy as a coot/loon

Lunatic behavior. The simile to the water bird dates from the sixteenth century, when John Skelton (Phyllyp Sparowe, 1529) wrote, “the mad coote, with a balde face to toote.” It is not known whether the craziness refers to the bird’s strange behavior in winter, when flocks of coots on a frozen pond sometimes fly wildly at one another, or to the senile behavior of the very old. (See also bald as a coot.) A related ornithological simile is crazy as a loon, probably derived from the weird loud cry of this bird. However, loony for “crazy” comes not from the bird but from lunatic, in turn related to the ancient belief that the phases of the moon (Latin luna) influence human behavior.
See also: coot, crazy, loon
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer
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References in classic literature ?
"Crazy as a loon," William sneered in open aside to the rest.
He took the measles when he was forty-five and was crazy as a loon with them, and the doctor ordered them to give him a dose of brandy.
Standouts from the album were Glory Of True Love, Crazy As A Loon and Long Monday, with superb harmonies from his guest, the marvellous Mindy Smith.