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