dumb as a rock

Terminally stupid.

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

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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.")

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dumb rocks

Rocks that are abnormally lacking in smarts.

"I didn't think it was possible, but that rock is dumber than most."

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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.

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Dumb ass a rock

Dumb as a rock combined with asshole and increased stupidity.
Most of people of this category do not understand the difference between Dumb as a rock and Dumb ass a rock

Guy 1: Your mom is as dumb as a rock.
Guy 2: Nope. She is ass dumb ass a rock.

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!