underjaw

He THINKS he's a Sheol of a farmer; thinks he's old Grayback from Wayback; but between you and me privately he don't know as much about farming as he does about running a kingdom -- still, whatever he talks about, you want to drop your underjaw and listen, the same as if you had never heard such incredible wisdom in all your life before, and were afraid you might die before you got enough of it.

(21.) Micrognathia is an abnormality characterized by smallness of the jaw, especially the underjaw. Stedman's Medical Dictionary 875 (5th ed.