Dumb as rocks 

Referring to someone that has no intelligence. No common sense. Someone who is just dumb, as rocks have no intelligence.

Neil: Dude you gotta check out this new car I got!!
Bob: What kind of car did you get??
Neil: A black one, it's awesome.
Bob: (thinks to himself; "this dude is dumb as rocks.")

Dumb as rocks by cpetkunas June 30, 2020

Dumb as a bag of rocks 

Extremely dumb.
Popularized on " The Big Bang Theory".

All the guys I knew growing up were as dumb as a bag of rocks.

Dumb as a bag of rocks by Judge dredd7 November 17, 2011

dumb as a box of rocks 

See "dumb as a bag of rocks" ... my deceased dad used to use the term alot.

You're about as dumb as a box of rocks ... /duh!

dumb as a box of rocks by Hippy420Lover53 May 14, 2018

dumb as a rock 

Terminally stupid.

The perp was dumb as a rock, and confessed in five minutes.

dumb as a rock by Octopod November 8, 2003