Pronounced "dokyun," it is a derogatory internet slang term that spread from 2ch.net, a Japanese BBS
. It mostly means dumbass or idiot, but it can also be used to refer to delinquents and violent, rough, uncultured, uneducated, socially inferior people. Though it used to be just 2channel slang, the term enjoys more widespread use now, and is even used on some English-language discussion boards.
Trivia, in case anyone's interested (culled from d.hatena.ne.jp and ja.wikipedia.org):
Mamii Ishida, a mysterious internet personality who was active around 1998 to 2000, is said to be the father of the term. It originated from "Mokugeki! Dokyun" (eyewitness dokyun), a Japanese TV show which ran from spring of 1994 to summer of 2002. The program showcased the deteriorated lives of people who had in their youth been "yankee" (delinquents), high school dropouts, and the like. (It was not spelled "DQN" in the title of the TV show.)
More recently, the term "DQN kigyou" (dokyun corporation) has been used to describe companies in Japan that exploit their employees by expecting unpaid overtime as a matter of course. Such companies are also referred to as "burakku kigyou" (black corporations).