Crazy as a coot/loon informal simile spoken
The phrase is often used jocularly.
Insane or weird.
Don't leave me alone with Mark- he's crazy as a coot!
If you think you can get away with that, you are as crazy as a loon.
Mr. John's bald, so why did you buy him a hair brush? You are as crazy as a coot!
Extremely stupid
Very stupid, or silly
Stupid or crazy
To be extremely stupid; not to be alert
If you're off your trolley, your ideas or behaviour are crazy or insane.
The simile to the water bird stems from the sixteenth century.