dumb as a post
(as) dumb as a post
Very stupid. You took apart the entire washing machine just to unblock the pump? I swear, sometimes you can be as dumb as a post, Larry. I used to think she was dumb as a post when we were kids, but she grew up to be a particle physicist.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
dumb as a postAMERICAN, INFORMAL
If 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!
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012