The normal Quenya word for “tooth”, appearing in The Etymologies written around 1937 derived from the root ᴹ√NELEK of the same meaning (Ety/NÉL-EK), where the final k became t. Its plural form nelki [nelci] indicates a stem form of nelk- [nelc-], since in most inflected forms the word would undergo the Quenya syncope and loose the second e. A variant form nelke (EtyAC/NÉL-EK) may reflect a reformation to align with those inflected forms.
Conceptual Development: In the Qenya Lexicon of the 1910s Tolkien had ᴱQ. nele (neli-) “tooth” under the early root ᴱ√NELE (QL/65), a word also mentioned in the contemporaneous Gnomish Lexicon (PME/65). In Early Qenya Word-lists of the 1920s Tolkien instead had ᴱQ. nyat “tooth” (PE16/136), but that seems to have been a transient idea. ᴹQ. nelet first appeared in Common Eldarin: Noun Structure from 1936 alongside variant nelke, but it was replaced by nele and then later nelke was deleted as well (PE21/56 and note #12). Both nelet and variant nelke were restored in The Etymologies (see above).
Primitive √nelek “tooth” was mentioned in Common Eldarin: Noun Structure from the early 1950s, but its Quenya derivatives were not listed.
Neo-Quenya: For purposes of Neo-Quenya, I’d use nelet (nelc-) “tooth” as the traditional form of this word, with nelcë as a modern variant with more regular inflections.
A prefix for “tri-” in The Etymologies of the 1930s under the root ᴹ√NEL “three” (Ety/NEL).
Conceptual Development: In the Early Qenya Grammar (EQG) of the 1920s, ᴱQ. nel meant “thrice” (PE14/51, 84).