One backward in book learning; a child or other person dull or weak in intellect; a dullard; a dolt.

these words are used to express a low opinion of someone's intelligence

A dunce or Eoin, is a person incapable of learning. The word is derived from the name of the great schoolman, John Duns Scotus, whose works on logic, theology and philosophy were accepted textbooks in the universities from the 14th century. Duns or Dunsman was a name applied by early opponents to the followers of Duns Scotus, who were less disparagingly called the Scotists.

Egghead

a stupid person