Ben Macdui is the highest hill in the upland of Mar. Significantly Ben Macdui isn't entirely within Aberdeenshire ; it straddles the boundary between the old counties of Aberdeenshire, and Banffshire. Its watershed marks the boundary between the two counties, with the boundary running through its summit cairn.
As a place name the derivation of Ben Macdui is obscure – apparently the most likely derivations are 'of the black pig', and 'of the sons of Duff' – depending how the name is interpreted, the components are there for both.
It's easy to speculate on a meaning, and many people have done just that, I've seen academic 'suggestions' (that's academic speak for speculation) elsewhere in the internet, and in books that the 'of the sons of Duff' derivation relates to this or that historical figure - but those academic speculations always depend on the key assumption that the name dates from the same period as their chosen historical figure.
Let's be clear on that point - I've seen no evidence, and I'm unaware of any, that the name Ben Macdui is more than a 'few' hundred years old. Indeed in Watson & Allan (1984) the authors raise the possibility of it being fewer than 300 years old by suggesting the derivation merely relates to the family of William Duff, 1st Earl Fife who bought Mar Forest, and the Aberdeenshire part of Ben Macdui in 1735.
Of course the spelling confuses the issue - Ben Macdui as it stands (so to speak) has no meaning - in capturing the name for posterity the spelling was mangled, and therein lies the difficulty - change the spelling slightly, and you get a different meaning. A good example of this ; a letter to Seton Gordon from William Watson dated December 23rd 1924 - reads :
I distinctly incline to believe that your friends the stalkers are right in saying that Ben Mac Dhui - a weird spelling! - means MacDuff's Hill, or something very like that. I am not familiar with the name in Gaelic, but I think it is Beinn Mac Duibh, which would mean ''Hill of the sons of Dubh'' in older Gaelic, while in modern Gaelic of the present day it would mean either that or ''Hill of the son of Dubh'', i.e. of Macduff ...
- William Watson
But William Watson was answering a loaded-question ; one loaded with a particular (if weird) spelling - given that spelling William Watson could only give the 'of the sons of Duff' answer. If he'd been given 'the other' spelling - he could only have given 'the other' answer - especially since he was 'not familiar with the name in Gaelic'.
Clearly - giving a particular spelling to a place name expert like William Watson will never resolve the question of what the name of the hill is - they can only tell you what the name you give them might mean. Surely we must first answer the 'spelling question', and the only satisfactory answer to the 'spelling question' can be found (if it exists) in the oldest spellings of the place name, in the oldest existing maps, and books.
The earliest published map I've seen of the area is based on the work of Robert Gordon, and was published in 1654 by Johann Blaeu. This map shows Carn-gorum M., Bini bourd M., and Bin-Avin., but there is no sign of Ben Macdui.
The earliest map I've seen showing the hill is from the survey of William Roy, dating to the period 1747-1755, and showing the spelling Ben Machdui - suggesting the 'of the sons of Duff' meaning.
There is a persistent rumour that an earlier map, or manuscript-map by Timothy Pont, or Robert Gordon shows something like Macduff's Cairn. I've looked at the online manuscript-maps of Timothy Pont, and Robert Gordon at the National Library of Scotland, and didn't find it, nor, when I queried library staff could they find it. However - the National Library of Scotland do have one manuscript-map by Robert Gordon showing the upper reaches of the Dé, and a tantalising label. It shows a flat-topped hill labelled soul bin MacDuff - this may be the origin of the persistent rumour, but there's little doubt this hill isn't Ben Macdui. This soul bin MacDuff is shown between two other labels : Potindeon, and Garrochoire Dee. I think this map is clearly showing Bod an Deamhain, Carn an t-Sabhail, and Garbh-choire Dhé in their relatively correct locations.
Carn an t-Sabhail means - hill of the barn, but this manuscript-map suggests an earlier meaning (something like) - barn-hill of MacDuff.
This manuscript-map only tells us two things we can be sure about : first - that MacDuff existed in a place name before the death of Robert Gordon in 1661 ; two - that Robert Gordon didn't write anything like Macduff's Cairn on this map.
Although it's possible there may be another manuscript-map by Timothy Pont, or Robert Gordon showing Macduff's Cairn it's not looking likely ; and this manuscript-map is a good suspect for being the origin of the persistent Macduff's Cairn rumor.
The earliest book I've seen referring to the hill is the account of the parish by Charles McHardy in the Old Statistical Account, dating to 1795, and showing the spelling Binn-na-muick-duidh - suggesting the 'of the black pig' meaning.
Since the Ben Machdui from the survey of William Roy, and the Binn-na-muick-duidh from the account of the parish by Charles McHardy both post-date the purchase of Mar Forest by William Duff, 1st Earl Fife in the 1730s - neither bring us closer to knowing the name of the hill before that purchase. The Duffs were evidently keen to 'prove' their decent from the ancient Earls of Fife - when offered an Irish Peerage in 1759, William selected Earl Fife, and Viscount Macduff to that end.
Without documentary evidence pre-dating the purchase of Mar Forest by William Duff we can't be sure if the name of the hill pre-dates his purchase in the 1730s, or is a subsequently 'invented' reference to his 'ancestors'.
The manuscript-map by Robert Gordon doesn't settle the argument either - but I'm unconvinced by the 'of the sons of Duff' meaning - that would make Ben Macdui the only hill in the whole of the Cairngorms not named for its appearance. For that reason, until I find evidence to the contrary, I'm inclined towards the 'of the black pig' meaning.