A silly old coot. Stupid as a coot. The coot is a small
water-fowl.
Bald as a coot.
The coot has a strong, straight, and somewhat conical bill, the
base of which tends to push up the forehead, and there dilates, so as
to form a remarkable naked patch.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894