The Collaborative International Dictionary of English v.0.48:

Doornail \Door"nail`\ (d[=o]r"n[=a]l`), n.
   The nail or knob on which in ancient doors the knocker
   struck; -- hence the old saying, "As dead as a doornail."
   [1913 Webster]

WordNet (r) 3.0 (2006):

doornail
    n 1: a nail with a large head; formerly used to decorate doors