Inspired by a comment here on reddit that jokingly claims Norwegians call themselves nordmenn (northmen) in order to accentuate their ties to the vikings.
The demonym for Norway literally means ‘northmen’ in all modern Scandinavian languages including Icelandic; but how and why – specifically in Norway itself as well as in its neighbouring kingdoms – did that term come to be associated exclusively with Norwegians? It makes sense that people from further down south in Europe would refer to them as such ('Lord deliver us from the wrath of the northmen’ and all), but that people living on approximately the same latitudes (e.g. Swedes and Icelanders) would adopt the same term seems less logical. I'm aware that there are multiple other instances in Scandinavian history of exonyms being derived from the location of a people’s settlements relative to that of the people naming them; Norwegians themselves were (at least in some cases) referred to as ‘eastmen’ in the Icelandic sagas, and according to Wikipedia the terms ‘northmen’ and ‘eastmen’ were regularly used to distinguish between inhabitants of different parts of Norway as recently as the 19th century.
So how did the term northmen, as opposed to, say, eastmen or austmenn, come to stick as a label for people from all over Norway? And this is just me speculating, but considering the prominence of medieval history in 19th century Norwegian national romanticist narratives, could the process have had anything at all to do with how the vikings (i.e. northmen) were perceived by descendants of their victims down south? Or was it purely a coincidence attributable to the way power happened to be distributed within Norway back when a Norwegian state was first being formed?
(Incidentally, the same wiki article also states – without citing any primary sources – that those defeated by Harald Hårfagre at the Battle of Hafrsfjord were known as ‘eastmen’ (austmennene) while those who fought on his side were the northmen (nordmennene), which I suppose would make a fine explanation considering the battle’s supposed significance in the ‘unification’ of Norway; but the actual contribution of Harald Hårfagre to Norwegian state formation is highly disputed anyway, and afaik there is still some controversy as to where exactly Harald was from and who exactly he came to rule over (for instance, the protagonist of Haraldskvedet, commonly identified as Harald Hårfagre himself, seems to have been styled ‘ruler of the eastmen’ – allvalds austmanna – even before his victory at Hafrsfjord) so I’m honestly not sure what to make of this information.)
As I discussed with /u/Platypuskeeper (and other redditors as well) before in Did the Vikings refer to themselves as “Northmen” within Scandinavia?, the (now) Norwegians had certainly called themselves as Norðmenn in Old Norse since around 900, but what this endonym meant in individual cases during the Viking Age(s) is not always easy to identify with.
AFAIK the 2nd oldest usage of the word, found in the stanza 3 of Hákonarmál, is the most telling example (also cited in the linked thread):
"{The sole slayer of jarls} [= Hákon] called on the Háleygir just as on the Hólmrygir; he went into battle. {The munificent terrifier of Island-Danes} [= Hákon] had the good support of the Norwegians; he stood under a helmet of metal." (English translation is taken from the official site of Skaldic Poetry of Scandinavian Middle Ages)
In this stanza, both inhabitants of Hålogaland (Háleygir) and of coastal Rogaland (Hólmrygir) are categorized under the Norwegians (Norðmenn), in contrast to the Island-Danes (Eydanir). Both protagonist [Håkon den gode (d. about 961 (?))] and the poet himself [Eyvindr skáldaspillir Finnsson] were also Norwegians, and especially the latter's historicity was probably one of the best attested in the 10th century Norway (As for the historicity of the person in 10th century Scandinavia in general, please also refer to my previous post, Was Hakon the Good a real person? More generally, how do we know people in the past really existed?). So, the usage of this poem must have been an endonym.
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Incidentally, the same wiki article also states – without citing any primary sources – that those defeated by Harald Hårfagre at the Battle of Hafrsfjord were known as ‘eastmen’ (austmennene)
Þorbjǫrn (hornklofi) says in St. 7 of Haraldskvæði that:
"‘‘Have you heard how the high-born king fought with Kjǫtvi inn auðlagði (‘the Wealthy’) there in Hafrsfjorden? Ships came from the east (austan), eager for battle, with gaping figure-heads and graven prow-boards.’" (English translation is taken from the official site of Skaldic Poetry of Scandinavian Middle Ages)
This [stanza of the] poem is almost all contemporary evidence of the composition of Harald's enemy at the battle, I suppose. Later traditions don't agree even the basic historical fact surrounding the battle like whether this Kjǫtvi was the (petty) ruler of Agder or not (Rowe in Mhaonaigh, Naismith & Rowe eds. 2020: 112f., esp. 113, note 14).
for instance, the protagonist of Haraldskvedet, commonly identified as Harald Hårfagre himself, seems to have been styled ‘ruler of the eastmen’ – allvalds austmanna – even before his victory at Hafrsfjord
Concerning this usage, while some historians are certainly annoyed like you (Cf. Rowe in Mhaonaigh, Naismith & Rowe eds. 2020: 113), I suppose you might overthink the connotation: since the poet [Þorbjǫrn (hornklofi)?] composed the poem describing the battle of Hafrsfjord first after the battle, the title of the protagonist employed in Haraldskvæði does not necessarily exactly corresponds with that before the battle. Alternatively, I don't know much about the poet's ethnicity, though his latest biographical note identifies him as a Norwegian - if he came from Iceland or other North Atlantic milieu, it would be no wonder he call all the inhabitants of current Norway as austmenn as 13th century Icelanders did.
the actual contribution of Harald Hårfagre to Norwegian state formation is highly disputed anyway, and afaik there is still some controversy as to where exactly Harald was from and who exactly he came to rule over.
Since about 1990, Claus Krag has been the almost only Norwegian specialist in the 10th century Norway, and he strongly argue against the traditional view (based on later saga traditions) of [fixed]: his Vestfold (eastern Norway) origin as well as the unification Process (rikssamlingen) beginning with the eastern Norway. Instead, he proposes that SW Norway (where Hafrsfjord was located) had been his power base, and also that his direct descendants had died out in course of the 10th century. AFAIK there has been almost no serious objection of this trend of revisionism in Norwegian historiography (since there is virtually no Norwegian historian specialized in the Viking Age except for him). My previous post on Did Harald Fine/Fairhair actually exist? might also be interesting to you.
Sorry for cluttering composition of the post. I'm also willing to answer any additional question.
Add. Reference:
(Edited): adds a sentence to clarify the different between the traditional and newer views of historians on "historical" Harald (probably not) Fairhair, but tangle-haired.