According to Tolkien, this is the proper noun form of the adjective adûn “west” (SD/435), though adûn is used as a noun in some examples as well.
Adûnaic
adûn
noun. west, westward
adûni
noun. the West
adûnakhôr
masculine name. Lord of the West
Son of Ar-Abattârik and the 20th ruler of Númenor, whose Quenya name was Herunúmen. In both languages, his name (somewhat heretically) means “Lord of the West” (LotR/1036, S/267). Its first element adûn means “west”, which implies that its second element means “lord”, but it isn’t clear whether this element is ✱akhôr or ✱khôr. I think that khôr is more likely, because it resembles the Primitive Elvish root √KHER “rule, govern, possess”, to which it may be related.
adûnayân
noun. language of Númenor
The language of Númenor, appearing in a 1965 document first published in Nature of Middle-earth in 2021 (NM/323). Prior to that point, various hypothetical words have been proposed for this language’s name, such as ✱Adûnaiyê (Andreas Moehn, EotAL), ✱Adûnâyê (Thorsten Renk, NBA/1), and Adûnâiyê (my own invention), modelled after Nimriyê “Elvish”.
adûnâ
noun. Númenórean, (lit.) Westerner
The Adûnaic word for the Númenóreans themselves, clearly related to adûn “west” (PE17/18, SD/240). It is attested only in plural forms Adûnâi (normal plural) and Adûnâim (subjective plural). The final vowel of its singular form must be a long â; if it were short, its plural form would be ✱✱Adûnî instead of the attested Adûnâi.
adûn izindi batân tâidô ayadda
[the] road west once went straight, (lit.) west straight road once went
The 11th phrase of the Lament of Akallabêth (SD/247), whose word order varied considerably in the different drafts of the text. The first two words are the adjectives adûn “west” and izindi “straight, right, true”. They modify the subject batân “road, path, way”, which is in the normal-case rather than the subjective. This is consistent with the grammatical rules of Lowdham’s Report, since the verb ayadda has a pronominal suffix a- “✱it” (SD/429).
The fourth word is an adverb tâidô “once, then”. The verb form yadda seems to be the past tense of #yad- “to go”, functioning here as a pluperfect (see SD/439). This makes sense in the narrative, since this sentence describes the previous state (the road west going straight to Valinor) while the next sentence uses the aorist tense to describe the current state (all roads being bent around the now-round world).
Tolkien’s glosses match the word order of the Adûnaic sentence: “west straight road once went”. This might be rendered in more ordinary English as “[the] road west once went straight”.
The previous (second draft) version of this sentence had more differences from the final version than any other sentence in the second draft (SD/312). It had a different word order, with the adjective izindi “straight” appearing directly before the verb, perhaps functioning as an adverb. It has ēluk instead of tâidô and the verb form yadda is missing the pronominal prefix a-. Unfortunately, Christopher Tolkien did not publish the English glosses for this sentence, so it is hard to decipher the meaning (if any) of these differences.
Adûnaic
Adûnaic
Adûnaic is an Anglicized name of the language.
A noun meaning “west(ward)” (SD/435, PE17/18). Tolkien stated that was “a loan word from Eldarin speech in the language of the Folk of Hador, from which Númenórean was later derived” (PE17/18). Probably it is derived from S. dûn “west”, as suggested by several authors (AAD/9, EotAL/NDU). Tolkien stated that it was an adjective and its proper noun form was adûni (SD/435), but adûn was used as a noun in phrases such as Bârîm an-adûn “Lords of the West” (SD/247).