Prehistoric Adûnaic was influenced by both Elvish and Dwarvish languages. In fact, it was influenced by Elvish at two difference stages: prehistorically before men came to the west and again when the Edain, ancestors of the Númenóreans, encountered the Elves of Beleriand (SD/414).

In the first stage, the prehistoric ancestors of the Edain encountered some of the Avari and adopted elements of their language, perhaps even deriving the majority of their speech from the Dark Elves (S/141). This early Elvish influence is disguised by millenia of linguistic evolution, first as the Avari languages separated from those of the Eldar, and then further as the prehistoric Mannish languages changed while men migrated westward.

Tolkien’s main example of such an early borrowing was the word Ad. minal “heavens, sky” (SD/414), which superficially resembles Q. menel and S. menel only because the isolated phonetic elements [m], [n] and [l] happened to undergo little change in any of the languages in question (in the case of Sindarin, the word was borrowed from Quenya). Other resemblances, such as Ad. bêth “expression, saying, word” and S. peth “word”, may have been due to coincidental convergent evolution.

At some later point in their wandering, the ancestors of the Edain met the Dwarves, who also influenced prehistoric Mannish languages (SD/414, WJ/317). Tolkien stated that “Khazadian [Dwarvish] ... resembles Adunaic phonetically and ... it is precisely at the points where Adunaic most differs from Avallonian [Elvish] that it approaches nearest Khazadian” (SD/414). This hints that the Dwarvish influence was later than that of the Avari, but we don’t have enough information about either Adûnaic or Khuzdul to examine this relationship in detail. One major point of influence, though, was the Adûnaic preference for triconsonantal-root words, a feature of Khuzdul as well (SD/415).

When the Edain reached Beleriand, they entered into a long period of contact with the best known Elvish languages, Sindarin and Quenya. These were the sources of the “late resemblances between the two tongues” described by Tolkien (SD/414), though they seem mostly to consist of borrowed vocabulary and some phonetic influences. Many of the Edain learned Sindarin, and it was commonly spoken in Númenor as well (LotR/1128). Therefore, the majority of the loan words were probably from Sindarin, though there is evidence of Quenya loan words as well, such as Ad. lômi from Q. lómë (SD/414).

The Adûnaic language of Númenor developed from the languages of the Edain (WR/159), most specifically from the language of the folk of Hador (S/147-8, PE17/18), since they were the most numerous of those peoples after the fall of Beleriand. In “Lowdham’s Report on Adunaic” (SD/413-440), Tolkien discussed two main periods in the development of Adûnaic: Primitive Adûnaic and Classical Adûnaic. Primitive Adûnaic is the language as it was spoken during the founding of Númenor, while Classical Adûnaic was the language as it was spoken just before Númenor’s fall. Primitive Adûnaic may have been almost the same as the Hadorian language spoken in the First Age, but we don’t have enough evidence to know for sure.

Adûnaic underwent some changes in the millenia of Númenor’s existence, though less than might be expected of a human language over that length of time. Perhaps its development was slowed by the conservative nature and long lives of the Númenóreans. The historical development from Primitive to Classical Adûnaic is discussed in the phonetics sections of these respective languages. After the fall of Númenor, Adûnaic became extinct, but some of its colonial dialects survived and developed into Westron (LotR/1128-9, WR/159, Let/425). These later developments are discussed in the historical development entry for Westron.