Hills between the river Anduin and the Dead Marshes (LotR/373), translated “Drear Hills” in Tolkien’s “Unfinished Index” of The Lord of the Rings (RC/334). It appears to be a combination of the plural of amon “hill” and muil, either an adjective meaning “drear” or a noun meaning “dreariness” (see its entry for further discussion).
Conceptual Development: In Lord of the Rings drafts from the 1940s, these hills went through many names: N. Duil Rain >> N. Emyn R(h)ain (TI/268), later N. Sern Lamrach >> N. Trandóran before finally settling on Ety Muil (TI/424).
emyn (pl. of amon “hill”), muil (Dor. “twilight, shadow, vagueness”)