1
general::
Also, deaf as an adder. Unable to hear or to listen, as in Speak louder, Grandpa's deaf as a post. The first simile has its origin in John Palsgrave's Acolastus (1540): "How deaf an ear I intended to give him . . . he were as good to tell his tale to a post." It has largely replaced deaf as an adder, alluding to an ancient belief that adders cannot hear; it is recorded in the Bible (Psalms 58:3-5).
American Heritage Idioms
2
general::
Phrase(s): *deaf as a post
deaf. (*Also: as ~.) • When my cousin was a teenager, she played her drum set without ear protection, and she was as deaf as a post by the age of twentyfive. • Mark can’t hear you even if you shout; he’s deaf as a post.
McGrawhill's American Idioms And Phrasal Verbs