dullard

Oh! Richard of Bury, I sighed, for a sharp stone from your sling to pierce with indignant sarcasm the mental armour of these college dullards.

Whereas some prodigies develop at an early age, Einstein did not exhibit any great genius as a young child. Some people thought he was a dullard.

Marion has long since stopped listening. "You dullard," she says now she can finally speak, "I'm your chief of Antimemetics."

Just look! The potatoes are all rolling out! You're a real dullard aren't you?

You're not very bright. You're actually quite a dullard.