dún

The Old Irish word dún (also dunn, dun) derives from the Proto-Celtic *dūno-, meaning an enclosure, a fortified place, or a hill-fort. This is one of the most productive roots in Celtic place-name formation: Proto-Celtic *dūno- gives Old Irish dún, Scottish Gaelic dùn, Welsh din or dinas (fortified town, city), Cornish dyn, Breton din — and in the Gaulish branch of Continental Celtic, it became the element -dunum that survives in the Latin-period names of dozens of Gaulish and Romano-British settlements: Lugdunum (Lyon, France), Vindunum (Le Mans), Camulodunon (Colchester, Camulodunum in Latin), Luguvallum (Carlisle), and Dunelmum (Durham). The Proto-Celtic root itself may connect to the Proto-Indo-European *dhewHno- (enclosed space, hill) or possibly to *dʰewbʰ- (deep, hollow), the sense being either a hollow enclosed space or an elevated enclosed space — the two principal forms of Celtic fortification (the hillfort commanding high ground and the enclosed lowland settlement).

In early Irish society, the dún was the residential and defensive stronghold of a local king or chieftain — typically a stone or earthwork enclosure on high ground, housing the household, cattle, and dependents of the lord. The geography of early medieval Ireland was organized around a hierarchy of such enclosures: at the top, the great royal sites like the Hill of Tara (Teamhair na Rí) and Cashel (Caiseal) and Emain Macha (Navan Fort); below them, the provincial and regional kings' dúnaigh; and below those, the homesteads of the tuath's freemen. The word dún in its Irish usage specifically denoted an enclosed defensive residence with political authority attached — it was not merely a settlement but a seat of power. The hundreds of Irish place names beginning with Dún- (Dundalk, Dungarvan, Dunlaoghaire, Dunmore) preserve this political geography in the modern landscape.

In Scotland, the same root produced the dùn as a type of Iron Age stone tower or fortification, and hundreds of Scottish place names carry the element: Dundee (from Dùn Dèagh, the fort of the Tay), Dunfermline (the fort by the winding stream), Dunbar (fort on the point), Dunblane (fort of Blaan), Stirling (possibly from dùn). The English place name element '-don' and '-ton' in some cases also derives from Celtic dūno- rather than from Old English dūn (a hill), though distinguishing the Celtic and Germanic contributions in individual place names is a matter of ongoing scholarly investigation. Winchester, originally Celtic Venta Belgarum, did not use the dun element; but Swindon (Old English Swīndūn, 'pig hill') derives from the Germanic cognate, while Dunedin (Edinburgh's Gaelic name, from Dùn Èideann) and dozens of New Zealand and Scottish places named by Scottish settlers carry the Celtic original.

The English color-word 'dun' — meaning a dull grayish-brown or yellowish-brown — derives from the same Old Irish and Celtic root by a different semantic path. Old English dunn meant dark-colored, dull-colored, and was used to describe the hides of horses and cattle — the muted, dusty brown-grey that was the natural color of many domesticated animals. This sense was borrowed or inherited from Celtic *duno- in its primary sense of the earth-colored, dusky quality of an earthwork fortification — the dun was the color of the ramparts, the earth and turf that formed the walls of the Celtic hillfort. The color word and the architectural word thus share a root in the earthy, undyed, non-vivid tone of natural building materials. In horse-breeding terminology, a 'dun' is a horse of a specific genetically determined coat color: a tan or yellow-grey body with dark points (mane, tail, and legs) and often a dorsal stripe — the ancestral coloring of many wild equid species.