A word appearing as {aitheb >>} eitheb “thorny” in Early Noldorin Word-lists of the 1920s (PE13/142). In the contemporaneous Early Noldorin Dictionary it was glossed “thorny, sharp” (PE13/158). It was an adjectival form of ᴱN. aith “thorn, †spear”.
Conceptual Development: Similar words in the Gnomish Lexicon of the 1910s include G. aithra “piercing, sharp; thorny” and G. aithog “thorny”, both based on G. aith “thorn” (GL/18). Gnomish Lexicon Slips modifying that document had {aithreg >>} G. aitheb “sharp, piercing” (PE13/108).
Neo-Sindarin: Since the noun [N.] aith/eith “spear-point” reappeared in Tolkien’s later writing, I would retain ᴺS. eitheb “thorny” for purposes of Neo-Sindarin, perhaps with an original sense “✱full of points”. However, I would not use this word for “sharp”, since there are other later words with this meaning like S. aeg.
An Early Noldorin word from the Lays of Beleriand of the 1920s that might mean “lightning” in the name ᴱN. Celeg Aithorn which might mean “Lambent Lightning” (LB/45), but this “translation” of the name might be an unrelated description instead.