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