Doornail is a term used to refer to something that is considered dead or lifeless. This term is commonly used in idiomatic expressions such as "dead as a doornail". However, there are several other synonyms that can be used interchangeably with doornail. Some of these synonyms include deceased, defunct, lifeless, inert, and non-functional. Each of these synonyms brings a slightly different nuance to the same idea, but they all convey the same basic concept of something that is no longer alive or active. It's important to have a variety of synonyms available to help keep your writing fresh and interesting, and to avoid using the same word repetitively.