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 that kid was dumb as a post, but he turned out to be a physicist.
See also: dumb, post
Farlex Dictionary of Idioms.

dumb as a post

AMERICAN, 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!
See also: dumb, post
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • (as) dumb as a post
  • (as) dumb as a fencepost
  • dumber than a doornail
  • dumber
  • dumber than a bag of hammers
  • swear down
  • swear
  • the truth, the whole truth, and nothing but the truth
  • I am so sure!
  • I'm so sure