The basis Sindarin word for “hill”. In one set of notes from around 1967, Tolkien said it could be applied also to any “lump, clump, mass” (PE17/93). In this same note Tolkien said it was “often applied to (especially isolated) mountains”, the most notable example being S. Amon Amarth “Mount Doom”. Its plural form emyn “hills” also appears in many names.
Conceptual Development: This word had a long history in Tolkien’s Elvish languages. It first appeared as G. amon “hill, mount, steep slope” in the Gnomish Lexicon of the 1910s (GL/19), where it was probably a derivative of the early root ᴱ√AM(U) “up(wards)”. ᴱN. amon “hill” also appears in Early Noldorin Word-lists from the 1920s again connected to am- “up” (PE13/137, 159), and was given as N. amon “hill” in The Etymologies of the 1930s as a derivative of the root ᴹ√AM “up” (Ety/AM²).
Its Quenya cognate Q. ambo was given as derivative of √AM “go up” in notes from 1967, but in other 1967 notes on the comparative, Tolkien coined some different roots as the basis for this S. amon “hill”, first √MAB “lump, mass” (PE17/90) and then √MBON, the latter being the basis for the alternate meanings “lump, clump, mass” mentioned above (PE17/90-93). Tolkien’s motivation for this change was that he wanted √AMA to have a new meaning “addition, increase, plus” to serve as the basis for the intensive.
Neo-Sindarin: For purposes of Neo-Sindarin, I would assume S. amon was derived from √AM “up”, since I prefer Q. an- for intensives, but it may have been influenced by √MBON and this was the reason for its alternate meanings “lump, clump, mass”.
pl1. emyn n. hill, lump, clump, mass, often applied to (esp. isolated) mountains. Q. umbo(n). FAmon Amarth