You might as well junk that car, the engine is dead as a door nail. Nails were once hand tooled and costly. When an aging cabin or barn was torn down the valuable nails would be salvaged so they could be reused in later construction. When building a door however, carpenters often drove the nail through then bent it over the other end so it couldn't work its way out during the repeated opening and closing of the door. When it came time to salvage the building, these door nails were considered useless, or "dead" because of the way they were bent.