Mouth of the river Anduin (LotR/400), translated “Mouths of Anduin” in Tolkien’s “Unfinished Index” of The Lord of the Rings (RC/350). This name is a combination of ethir “river mouth” with the river name.
Conceptual Development: In Lord of the Rings drafts from the 1940s, this name first appeared as N. Ethir-andon, soon revised to N. Ethir-anduin, matching changes in the river name itself (TI/298).
A noun for the mouth of a river or a river delta, glossed “outflow” in the Unfinished Index of The Lord of the Rings (RC/350; Ety/ET). It was a combination of primitive ✶et “out” and S. sîr “river” (SA/sîr; Ety/ET), where ts > th.
Conceptual Development: The word and its derivation first appeared as N. ethir in The Etymologies of the 1930s (Ety/ET).