The Etymologies of the 1930s had ninn as a cognate to ᴹQ. ninde “slender”, both derived from the root ᴹ√NIN-DI “fragile, thin” (Ety/NIN-DI).
Neo-Sindarin: I would adapt this word as ᴺS. nind as suggested in HSD (HSD), in keeping with the notion that “nd remained at the end of fully accented monosyllables” (LotR/1115).
Conceptual Development: Early Noldorin word-lists of the 1920s had ᴱN. minedh glossed “fine, slender” (PE13/150) or “thin, fine, small” (PE13/164), derived from primitive ᴱ✶miniyā.
An adjective in The Etymologies of the 1930s glossed “fine, slender” derived from ᴹ✶terēwā “piercing, keen” under the root ᴹ√TER “pierce” (Ety/TER).
Conceptual Development: Early Noldorin word-lists of the 1920s had ᴱN. tlui “slender” derived from ᴱ✶loise and cognate to ᴱQ. toloise “rounded, cylindrical” (PE13/154).