What does dead as a door nail mean?

"When doors were built using only wood boards and hand forged nails, the nails were long enough to dead nail the (vertical) wooden panels and (horizontal) stretcher boards securely together, so they would not easily pull apart. This was done by pounding the protruding point of the nail over and down into the wood. A nail that was bent in this fashion (and thus not easily pulled out) was said to be dead. Thus the expression - dead as a doornail."