In this meaning we almost never say doornail by itself. We say someone or something is “dead as a doornail” — completely dead, with no life left at all. It is a set phrase, a little old and a little funny. It does not mean “dead like a nail.” You can use it about a person, an animal, or even a machine that will never work again. You would not use it when you need to be gentle, like in a hospital. A doornail was first just a nail used in a door; the phrase got used so often that this sense now just means “totally dead.”

死透了（幾乎只出現在 dead as a doornail 這句裡）