From a 2006 episode of the tv series Psych, “Death is in the Air”:
Is she dead?
Shawn: As a bag of rocks.
Gus: That’s “dumb as a bag of rocks”.
(Or “dead as a rock/stone”. But not part of each.)
This idiom blend seems to be a genuine invention of Psych‘s writers. No other ghits for it.
Three classic idiom blends, illustrated here (with references): rocket surgery in “It’s not rocket surgery”, under the eight ball, another kettle of worms. A few more from net discussions of the phenomenon: by far and away, don’t take lightly to, add up the math, a kick in the bucket, knock the wind out of your sails.
(Hat tip to Victor Steinbok.)