"DEAD AS A DOOR NAIL"
Nails were once hand-tooled and costly. When someone tore down an aging cabin or barn he would salvage the nails so he could re-use them in later construction. When building a door, however, carpenters often drove the nail through then bent it over on the other end so it couldn't work its way out. When it came time to salvage, these bent "door nails" were considered useless or "dead."