An adjective appearing as teren or terene “slender” in The Etymologies of the 1930s, derived from ᴹ✶terēn(ē) under the root ᴹ√TER “pierce” (Ety/TER). The Qenya Lexicon of the 1910s had ᴱQ. teren “lissom, lithe” under the early root ᴱ√TEŘE [TEÐE] also having to do with piercing, but Tolkien marked ᴱQ. teren with a “?” (QL/91).
Qenya
-rin
suffix. -ian, racial-adjective, language
-voite
suffix. adjective suffix
ninde
adjective. slender, slender, *thin
-ya
suffix. adjective suffix
loa
noun. year
teren
adjective. slender, slender; [ᴱQ.] lissom, lithe
terene
adjective. slender
tol eressea
place name. Lonely Isle
yén
noun. year
A word for “slender” in The Etymologies of the 1930s under the root ᴹ√NIN-DI “fragile, thin” (Ety/NIN-DI).
Neo-Quenya: For purposes of Neo-Quenya, I would also use this word for “✱thin” based on the meaning of its root.