What is dumb as a doornail?

The phrase "dumb as a doornail" is used to describe someone who is very stupid or completely devoid of intelligence. The origin of the phrase is not clear, but it has been in use since at least the 16th century.

One theory is that "doornail" refers to the large-headed iron nails that were used to hold a door in place. Once the nail was hammered into the wood, it was considered "dead" or unusable for any other purpose. Similarly, a person who is "dumb as a doornail" is seen as being as lifeless and useless as a dead nail.

Another theory is that the phrase comes from the practice of using doornails to fasten a coffin shut. If a person was truly dead, they would be just as "dumb" as the nail that sealed their coffin.

Regardless of its origins, "dumb as a doornail" is still a commonly used expression today.