The term 'Dunce' was, and still is, a technical term.

The source of the term is an English 'scholar' named Duns Soctus who died in 1306. He wrote a book that covered all the knowledge in the known world and it became the accepted standard in English education.

Once you had mastered Duns' book you were qualified to be addressed as a 'Dunsman'. This was considered the highest level of education attainable and was a necessary title to hold if one desired a teaching position. Dunsmen, and only Dunsmen, were entitled to wear the pointed cap that signified their rank in society.

By the 1500's it had become clear that not just much of what Duns' had put down was wrong but that all of it was. But Dunsmen continued to teach from Duns' book as they had spent their lives working towards their positions and did not wish to give them up.

By the 1600's all the Dunsmen had died out. The Dunce Cap lived on and became the symbol we know today: That of a person who is wrong and yet continues to make the same mistakes even after they have been corrected.

The term 'Dunce's table' first appears in print in 1624.

The term 'Retard' is a slur and has always been a slur. The term 'Dunce' is a description and has always been a description.

To quote the ancient Greeks: If the shoe fits, wear it.