Nothing Scandinavian about Duff, it's from the Scottish Gaelic "dubh", meaning 'black'. Related to Irish Duffy.
Yes, insofar as the Clan histories are concerned.
However, I would refer you to:
https://ia800203.us.archive.org/4/items/cu31924029805771/cu31924029805771.pdfBarber, Henry, M.D. (Clerk).
British Family Names: Their Origin and Meaning, with lists of Scandinavian, Frisian, Anglo-Saxon and Norman Names. (London: Elliot Stock, 1894.)
The author has stated his sources, which included "Buchanan's Scottish Surnames".
From the
Introduction the author states: "The chief object has been to avoid anything like guess-work or fancy interpretation, and to seek for a definition among such sources as seemed likely to supply it, so that, when a word could not be referred to any reasonable origin, it was put on the shelf until time, experience, and discovery should verify it.
My Duff ancestors were entrenched in Edinburgh back to the early 1600s where they were found as armourers, gunsmiths, and cutlers. So, whether they were descended from or sought shelter under the auspices of the Duff clan I do not know. Nor are there any documentary sources available that could prove this point for me. So, while I find the Clan book writings interesting, I have to question the origin of the surname prior to even the commencement of the history of the clans.
One very rarely is able to determine whether an ancestor bearing a Clan surname was an actual biological descendant of that Clan Chieftain or whether that ancestor had simply sought shelter from the clan and took the chieftain's surname.
Were my Duffs Picts or of Viking descent? No one can answer this. My DNA, however, certainly points to quite an involved descent from Scandinavian individuals and the author above-noted has apparently found sufficient evidence during his research to associate the surname Duff with that of Scandinavian origin. So, for now, I will hang my hat in the Scandinavian origins corner.