In some cases, it is an Anglicised form of the Gaelic Ó Duibh ("descendant of Dubh"), Mac Giolla Duibh ("son of the servant of Dubh"), Mac Duibh ("son of Dubh").
The surname Duff is also sometimes a short form of Duffin (when of Gaelic origin), and MacElduff (from Mac Giolla Duibh), and Duffy (a name with multiple origins).
[1] The Gaelic dubh ("black", "dark") is a word-element which forms a part of several Gaelic names.
[1] Le Duff is the gallicized variant of the Breton surname An Du which means The Black.
In the British Royal Family: Scottish aristocrats: