Mimoun Aoudal, I wanted to know the reason behind this expression myself, so I did some research. The most plausible explanation I found is this one: If you hammer a nail through a piece of timber and then flatten the sharp end over on the inside so it can’t be removed again (a technique called clinching or dead-nailing), the nail is said to be dead, because you can’t use it again. Doornails, which are very long, would very probably have been subjected to this treatment to give extra strength in the years before screws were available. Nails were hand made in the olden days and comparatively expensive, so they were often re-used. So, apparently "dead as a doornail" means not only lifeless, but also of no use to anyone in the future.

Utterly lifeless. Here is a good source for finding the meanings of idioms. http://www.idiomdictionary.com/

Totally dead. Most certainly dead. Dead without a doubt.

moorche answered it correctly, I just wanted to add something so you know... In English, you will see these kinds of sentences a lot, they are called Similes. It is a comparison using "like" or "as". So, if you read a sentence with two nouns (person, place, or thing) and they are related together using one of these two words, then it is usually an idiomatic phrase. We use similes in our slang, and they can be made up with almost anything, for example: "Your head is as big as a watermellon." -You are comparing "head" with "watermellon" because watermellons are big. As you listen to more English you will start to pick up on when these are being said. Hope I helped a little =)

Completly dead.