http://www.rampantscotland.com/clans

From the Gaelic words "donn" (brown) and "cath (warrior) the name Duncan was initially a forename and there were two Kings of Scotland with that name in the 11th century, Duncan I and Duncan II. The surnames Duncan and Robertson or Clann Donnachaidh have similar origins, being descended from the ancient earls of Atholl and took their name from Donnachaidh Reamhar who led the clan at the Battle of Bannockburn (June 24, 1314). The Duncans owned lands in what is now the county of Angus 　but it is found over much of Central Scotland.

Adam Duncan from Dundee became a naval commander and defeated the Spanish at Cape St Vincent in 1780 and the Dutch fleet at Camperdown in 1797, one of the most famous victories of the British Navy for which he was created Viscount Duncan by George IV in 1800. The Rev Henry Duncan founded the first Savings Bank in 1810 in Ruthwell, Dumfriesshire.

Duncan was the 36th most frequent surname at the General Register Office in 1995.

The motto of the clan Duncan is "Disce pati" ("Learn to suffer").