as dumb as a box of rocks

buffoonish dopy thick-skulled mentally retarded analphabetic bookless softheaded idiotical inerudite hebete

scholastic able read conversant savant capable

as dumb a box of rocks

as dry as snuff as dull as dishwater as dumb as a bag of hammers as dumb as a doorknob as dumb as a doornail as dumb as a post