Noldorin
thlinn
adjective. fine, slender
thlinn
adjective. fine, slender
thlind
adjective. fine, slender
trîw
adjective. fine, slender
thlinn
adjective. fine, slender
thlinn
adjective. fine, slender
thlind
adjective. fine, slender
trîw
adjective. fine, slender
The Gnomish Lexicon of the 1910s had G. thlind “fine, slender” (GL/73), likely related to ᴱQ. sild(r)a “slender” derived from the early root ᴱ√SḶŘḶ [ÞḶÐḶ] (QL/84). The Etymologies of the 1930s also had N. thlinn, thlind “fine, slender”, but there it was derived from the root ᴹ√SLIN (Ety/SLIN) with the initial *sl* becoming *thl* as was generally the case in Noldorin of the 1930s.
Neo-Sindarin: In Sindarin of the 1950s and 60s, initial *sl* became voiceless *lh*, so I would adapt this word as ᴺS. lhind as suggested in HSD (HSD).