A noun in The Etymologies of the 1930s glossed “thorn” under the root ᴹ√EK (Ety/EK). In The Etymologies this root was glossed “spear”, but I think this gloss applied only to the extended form of the root √EKTE, because elsewhere √EK had other glosses like “sharp, (sharp) point, thorn” (WJ/365; VT48/25; PE22/127).
Conceptual Development: In the Gnomish Lexicon of the 1910s, G. eg merely meant “point” (GL/32) while G. aith was “thorn” (GL/18), both from the early root ᴱ√EKE or ᴱ√EHE having to do with points (GL/31; QL/35). In Gnomish Lexicon Slips this became G. aithr “thorn” which also had an archaic sense “†sword” (PE13/108), a word that also appeared as ᴱN. aithr “thorn, [archaic] †sword” in Early Noldorin Word-lists of the 1920s (PE13/136).
From a passage inadvertently omitted from the published version of The Silmarillion (LR/301), this name is a combination of eithel “fountain” and nínui, apparently an adjective meaning “✱tearful”.