The phrase has also been used in literature to describe the state of being dead. In Shakespeare's play "Romeo and Juliet," the character Mercutio uses the phrase to describe the death of Tybalt. He says, "Tybalt, liest thou there in thy bloody sheet? Death, that hath suck'd the honey of thy breath, / Hath had no power yet upon thy beauty: / Thou art as fair in death as thou wast in life, / But dead as a doornail, as dead as a doornail!"
The phrase has also been used in more recent times. In the 1960s, the Beatles released a song called "A Day in the Life" that included the line "He blew his mind out in a car / He didn't notice that the lights had changed / A crowd of people stood and stared / They'd seen his face before / Nobody was really sure / If he was from the House of Lords." This line is a reference to the phrase "dead as a doornail," and it suggests that the man who died in the car accident was so dead that he didn't even notice that the lights had changed.
The phrase "dead as a doornail" is a powerful simile that is used to express the finality of death. It is a reminder that death is a natural part of life, and that it will come to us all eventually.