Deaf as an adder. (See below, Deaf adder.)
Deaf as a post.
Quite deaf; or so inattentive as not to hear what is said. One
might as well speak to a gate-post or log of wood.
Deaf as a white cat.
It is said that white cats are deaf and stupid. None so deaf as
those who won't hear.
The French have the same locution: “Il n'y a
de pire sourd que celui qui ne veut pas entendre.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894