Thick as a plank/as two short planks/as a brick
Wood or timber as an inanimate object has long been used as a simile for lack of intelligence or stupidity in humans e.g. blockhead or wooden-headed. For even more emphasis, a person may be described as thick as two short planks. The word thick has been used in the context of thick-headed, crass or stupid since the 16th century but thick as a plank and thick as two short planks are relatively modern expressions, dating from the 1950s and are thought to have originated in the British military. Thick as a brick is a variation of the same theme and also dates from the mid-20th century.