Nails used to be quite valuable, so much so that people would remove...
· Comment — Gerry Mander's post · View on Facebook · Markdown source
Nails used to be quite valuable, so much so that people would remove them and take them with them when they moved. However no one wants a door that can be easily dismantled, so they would bend the nails in half to prevent them from being removed or reused. Hence, "dead as a doornail'.