I am a frequent contributor at the English Wiktionary, largely contributing Irish translations of English words. Recently, someone added a request for the etymology of the Irish word “dún”, meaning “fort” and a common element in Irish placenames–such as Dún na nGall (“Fort of the Foreigners”), the Irish on Donegal. This is a request easily filled. The earliest attested form of the word is the Old Irish “dún”, carrying the same meaning. I decided I’d see what else I could dig up, though.
There’s an online etymological dictionary for Celtic languages available at http://www.indo-european.nl/. It has some apparent encoding problems and does not source its data, but what I’ve been able to compare of its Proto-Indo-European roots usually matches up with other sources. I fed it “dún” to see what it had to say. It traces the word back to the Proto-Celtic root *dūno- and lists a few cognates in other Celtic languages. Then, however, it gets interesting.
Matasović provides both a Proto-Indo-European form *dʰuHno- (“enclosure”) and a cognate in none other than Old English, namely “dūn”. Come the Great Vowel Shift ca. 1500 and you end up with Modern English “down”, a now-archaic word meaning “hill” (and still found in some placenames). The Oxford English Dictionary also suggests that an inflected form meaning “off the hill” is the origin of the adverbial form we know and love.
Things do get more interesting from there. Matasović also suggests that English “town” and German “Zaun” (“fence”) are related to this word through a Proto-Germanic form *tūno-, the Modern English coming from the Old English “tūn”. This would, of course, suggest that in Proto-Germanic the forms *dūno- and *tūno- existed side by side. To explain this, Matasović suggests that *tūno- is borrowed from Proto-Celtic *dūno- somewhere along the line. To explain this, we’ll look more closely at the Proto-Indo-European root and a few sound changes that occurred in languages derived from Proto-Indo-European.
As mentioned above, the Proto-Indo-European form Matasović gives is *dʰuHno-. Proto-Germanic underwent a series of sound changes, including the change from PIE *dʰ > *d. Presumably this change happens in Proto-Celtic as well, though I haven’t read any research on this. (It seems both also have PIE *uH > *ū. I’m not clear on what exactly happens here. The H itself could be one of three proposed sounds in PIE, but I’m too unfamiliar with PIE to speculate much.) However, Proto-Germanic also undergoes a change of PIE *d > *t. If Matasović is correct, this would indicate the following order of events: Proto-Celtic changes *dʰ > *d, Proto-Germanic borrows Proto-Celtic *dūno-, Proto-Germanic changes *d > *t and *dʰ > *d. The first of the Proto-Germanic sound changes would have to happen either earlier than or contemporaneously with the second for the sounds to remain separate.
There’s more, of course. Thinking of Old English “dūn” and the semantically similar Modern English “dune”, I decided to investigate the provenance of that word as well. The OED’s earliest citation for the word is 1790 and its etymology section indicates it comes from Modern French “dune”. It also indicates that this latter is cited in Old French in the 13th century, Old French having borrowed it from Middle Dutch “dûne”. This in turn derives from Old Dutch “dûna”. Wiktionary goes on to suggest that this is a borrowing from a Celtic language, while the Online Etymological Dictionary suggests specifically that it is from Gaulish *dunom (Gaulish being a Celtic language spoken on the Continent). Of note, neither Wiktionary nor the Online Etymological Dictionary give citations for their suggestions and neither is necessarily reliable. (Though a niggling point, that other sources consistently give *dunum as the reconstruction of the Gaulish reflex of *dūno- may demonstrate why I distrust Etymonline.)
It is possible that this form is a borrowing from a Celtic language. It also seems to me (though unbolstered by research or clear knowledge of the various changes that occurred in the languages involved) that Old Dutch could simply have derived its form from the same Proto-Germanic root that yielded Old English “dūn”. Then again, that root could itself be an even earlier borrowing from Proto-Celtic. It’s possible that some research into Indo-European languages outside of Proto-Celtic and Proto-Germanic would reveal a reflex that indicated that the PIE root did exist. Further research is, as always, necessary.