Pictured above is a young man, likely at school, who is in trouble -- given what he's written on the board, he appears to have been misbehaving. His punishment, beyond writing lines attesting to what he needs to do better, is embarrassment: he's to sit in a chair, for all to view, wearing a pointed hat with the word "dunce" written on it. The "dunce cap," as that hat is commonly referred to, is a signal to everyone who sees it that the wearer is, at least in the view of the relevant authority, an idiot. 

That's probably not fair to a guy named John, who happened to be the original dunce.

John Duns Scotus -- not the kid pictured above -- was born in 1265 or 1266. His name tells us his birthplace: Duns is a town in Scotland not too far from Edinburgh (here's a map) and his name, effectively, means "John from Duns." While we don't know much about his early life, we know that in adulthood, he became one of the most respected philosopher-theologians of his time. (His Wikipedia entry groups whim with Thomas Aquinas, among others.) His teachings became a branch of philosophy/theology called Scotism; followers of Scotism were called Scotists or, at times, Dunsmen or Dunses. 

And they may have worn hats. According to The Straight Dope, "one of the more mystical things Duns accepted was the wearing of conical hats to increase learning. He noted that wizards supposedly wore such things; an apex was considered a symbol of knowledge and the hats were thought to 'funnel' knowledge to the wearer." There's some question as to the accuracy of that claim, but John Duns Scotus himself, seen here, was often depicted with an elongated cap (no pointy top though), so there's likely some association with dunce caps and Duns Scotus himself. But either way, a century or two after John Duns Scotus died in 1308, being a follower of Duns wasn't a good thing.

When the Renaissance began to take foot in Europe in the 14th century, Europeans began studying the history of ancient Greece and ancient Rome, two civilizations that by and large existed before Christianity. Philosophers of the time took views of the world that often clashed with the theologically-driven orthodoxy of existing schools of thought. Many saw Scotism as particularly unwavering and unwilling to question their prior beliefs -- and that led to namecalling. As Atlas Obscura explains, Duns Scotus's " intensely analytical writings and labyrinthine logic came to be seen as overly complex, and increasingly in conflict with the more humanistic views emerging with Renaissance thought. The remaining Dunsmen, who continued to devote themselves to Scotist thought, began to be thought of as hopelessly behind the times, or just plain stupid." The humanists had no problem telling others that Dunsemen were idiots, and "Duns" quickly became a term used to describe an unsophisticated person who had no real interest in scholarship.

The slang term became the non-slang word "dunce" over time and is still used today (although decreasingly so). John Duns Scotus, who was seen as brilliant in his day, would have seen his reputation slide as a result of the smear campaign. But in recent years, the Catholic Church has taken at least one step to address that: Pope John Paul II beatified Duns Scotus in 1992.