A word appearing as N. nêl “tooth” in The Etymologies of the 1930s derived from the root ᴹ√NELEK of the same meaning (Ety/NÉL-EK). It was one of a rare set of primitive words with final k, which was lost in very early stages of the language, to produce ON. nele (PE21/56). Its ON. plural was neleki, which produced nelig in the modern form of the language. Since the plural preserved the stop lost in the simplex, an alternate form N. neleg was restored from the plural.

This longer form neleg appeared as an element in a number of names from Lord of the Rings drafts from the 1940s: {N. Neleg Thilim “Gleaming Tooth” >>} N. Neleglos “White Tooth” (WR/106) and N. Nelig Myrn “Teeth of Mordor” (WR/113). None of these names made it into the published version. However, primitive √nelek “tooth” was mentioned as the basis for Ancient Telerin nele “tooth” in notes from the early 1950s, again with loss of final k and plural neleki (PE21/71-72).

Neo-Sindarin: For purposes of Neo-Sindarin, I’d use neleg as the ordinary word for “tooth”, and limit nêl to the final element of compounds.