Dumb as a post means someone who is very stupid, like a fence post.
In American informal use, if you are describing someone as dumb as a post, you are saying they are very stupid.
Example: She’s one of the most famous physicists in the country, and she’s dumb as a post!
There are other variations of the phrase: “Dumb as a doorknob,” “Dumb as dirt,” and “Dumb as a doornail,” but the list is immense. All of these are similes, a figure of speech involving the comparison of one thing with another of a different kind, as a description using the words like or as.
Another weird phrase, because how can an inanimate object be stupid?