as dumb as a doorknob

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

scholastic read conversant savant capable literary

as dumb a doorknob

as dull as dishwater as dumb as a bag of hammers as dumb as a box of rocks as dumb as a doornail as dumb as a post as dumb as a sack of hammers