The surname `Dunn' is common in the eastern Lowland regions of Scotland, and is said to be derived from the land of Dun, a fort in Angus not far from Montrose.
The Gaelic word `donn', meaning `brown,' was and is as common a nickname in the Highlands of Scotland as it is in several countries (in their respective languages) in the world. Brown eventually became a Scottish surname in its own right; so did Donn or Dunn, with Dunn becoming the more prominent spelling.
The variation `Dunn' was an early form of the word, meaning in English, `dark' or `swarthy'.
The surname `Dunn' is also popular in Ireland.