Duncan is a surname. For the etymology of the surname Duncan this web page cites: Dictionary of American Family Names. Another opinion is that the Gaelic Donnchadh is composed of the elements donn, meaning "brown"; and chadh, meaning "chief" or "noble". In some cases when the surname originates in (Sligo) Ireland it is an Anglicised form of the Gaelic Ó Duinnchinn, meaning "descendant of Donncheann". The Gaelic Donncheann is a byname composed of the elements donn, meaning "brown-haired man" or "chieftain"; and ceann, meaning "head". The surname Duncan is represented in Scottish Gaelic as MacDhonnchaidh.