The man behind "dunce" was one of the most respected thinkers of the Middle Ages. He is remembered through an insult.

John Duns Scotus was a Franciscan theologian and philosopher, born around 1265 in Duns in the Scottish Borders. He died before the age of fifty. His school, Scotism, opposed some of the teachings of Thomas Aquinas and supported the doctrine of the Immaculate Conception. Etymonline says his followers dominated the universities until the Reformation. The Church beatified him in 1993.

The reversal happened in the 1500s. Etymonline says that in the 1520s humanists reacting against medieval theology cast Scotus as the model of the hair-splitting scholastic, and "Duns disciple" and "Duns man" appear in that decade. The Saturday Evening Post adds that die-hard Scotists, also called Dunsmen, fought against the waves of the Renaissance and the Reformation. "Duns" turned into a general insult for a stubborn or sophistical opponent.

According to Wikipedia, the Oxford English Dictionary cites mid-16th-century uses for a follower of Scotus, a pedant or a fool, and Richard Stanyhurst said the word came from the philosopher's name. Etymonline dates "dunce" itself to the 1570s, when it was applied to any poorly performing student, which gave us the modern sense. The Saturday Evening Post says "Duns" later became just "duns" and then "dunce." The dates therefore vary a little by source, from the 1520s for "Duns man" to the 1570s for "dunce."

The dunce cap came later, and its early history is murky. Etymonline dates "dunce's cap" to 1792, while Wikipedia records the term as early as 1791 and cites a picture in a 1727 edition of The New England Primer. Wikipedia gives Charles Dickens's The Old Curiosity Shop in 1840 as the first literary use. The Saturday Evening Post calls it the first recorded mention of the cap, which conflicts with the earlier dates. The tale that Scotus wore a pointed hat to channel knowledge into his brain is repeated on Wikipedia. The Saturday Evening Post, however, says there is little evidence he wore or invented the hat, and he is missing from the roughly six surviving portraits.

Whatever the truth about the hat, the word is a clean example of a reputation reversed by rivals. The scholar whose followers were once the establishment gave his name to the person sitting in the corner of the classroom.