Coot

Tom can't tell a coot from a moorhen.

A silly coot

An old coot

A rich coot

Once more he thought aloud. / "Tom wouldn't lie to me, so it wasn't gin. Now, I wonder. I wonder if that old coot has got what they call 'delusions of grandeur'?"

Your clerks would come in and see you aswingin' and aswayin' there and one of them would say: "Well, the old coot's hung himself!"

“You'll be able now to give it as your considered opinion that [Wilbert Cream] is as loony as a coot, Sir Roderick.” A pause ensued during which [the psychiatrist] appeared to be weighing this, possibly thinking back to coots he had met in the course of his professional career and trying to estimate their dippiness as compared with that of W. Cream.