(as) dumb as a post
Very stupid. Are you dumb as a post? Why did you think you could fix the washing machine on your own? I used to think that kid was as dumb as a post, but he turned out to be a physicist.
dumb as a post
AMERICAN, INFORMALIf you describe someone as dumb as a post, you mean that they are very stupid. She's one of the most famous physicists in the country, and she's dumb as a post!