A dunce cap, also variously known as a dunce hat, dunce's cap, or dunce's hat, is a pointy hat. In popular culture, it is typically made of paper and often marked with a D or the word "dunce", and given to schoolchildren to wear as punishment by public humiliation for stupid or lazy behavior.While this is now a rare practice, it is frequently depicted in popular culture such as animated television series. Such headwear is most prevalent in Western culture but achieved a certain prevalence in modern China in connection with various elements of the communist movement.A very similar practice on the European continent was a paper headdress known as donkey's ears, as a symbol of 'asinine' stupidity.
Origins
The word "dunce" was originally a reference to Bl. John Duns Scotus, a 13th century scholastic theologian, whose books on theology, philosophy, and logic were University textbooks. His followers, termed "Dunsmen" or "Dunses", were later challenged about their system of hair-splitting a
Source:
http://en.wikipedia.org/wiki/Dunce