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).
[His.] ed (“out”) + sîr (“flow, river”), and (“long”) + duin (“long and large river”) David Salo: ”dh and mh were liable to revert to d and m when they came to follow a nasal after syncope” TolkLang message 19.31.