dumb as a door 

someone that is so stupid that can only open and close, not literally.

Farin is dumb as a door; he doesn't have a clue what it waits for him.

dumb as a door by Lxiaoqi April 7, 2018

dumb as a doornail 

someone who is just stupid,
and doesn't even know what doornail means anway so isn't really insulted by the term anyway.

"are you saran wrap?"
"who's that?"
"wow your dumb as a doornail"

dumb as a doornail by ashleyyaddiction September 7, 2007