“The wicked go astray as soon as they be born, speaking lies. Their poison is like the poison of a serpent: they are like the deaf adder that stoppeth her ear; Which will not hearken to the voice of the charmer, charming never so wisely.”
Psalms, lviii, 3-5.
Or, in the language of the Early English Psalter (about 1300), “Als of a neddre als-swa yat (that) stoppand es his eres twa.”
The allusion is to the ancient Oriental belief that certain serpents were able to protect themselves against being lured by the music of charmers by stopping up one ear with the tip of the tail and pressing the other firmly to the ground.
Thus the phrase “deaf as an adder”.