A noun appearing as N. thlingril “spider” in The Etymologies, probably a feminine form of N. thling “spider, spider’s web, cobweb” (Ety/SLIG). Christopher Tolkien said the r was uncertain.
Neo-Sindarin: Since initial sl- became lh- in Sindarin, most Neo-Sindarin writers adapt this word as ᴺS. lhingril, as suggested in HSD (HSD). This word is somewhat questionable, since in later writings √ungu- was the basis for spider words (PE22/160). I think ᴹ√SLIG may be reinterpreted as having to do with threads, and as such I think ᴺS. lhingril can be retained as originally meaning “(female) web spinner”. However, I recommend later S. ungol as the more common Sindarin word for “spider”.
A noun appearing as N. thlê “fine thread, spider filament” in The Etymologies of the 1930s derived from ᴹ✶(s)ligā under the root ᴹ√SLIG having to do with spider words (Ety/SLIG).
Conceptual Development: A possible precursor is G. lind “twine” from the Gnomish Lexicon of the 1910s (GL/54), apparently based on the early root ᴱ√LI+ya “unite many as one” (QL/53).
Neo-Sindarin: In Tolkien’s later writings, √ungu- became the basis for spider words, but I retain ᴹ√SLIG assuming it has to do with threads and twining to salvage words from the 1910s and 30s. Most Neo-Sindarin writers adapt this word as ᴺS. lhê “fine thread, spider filament”, for example as suggested by Hiswelókë’s Sindarin Dictionary (HSD), because in (Old) Sindarin of the 1950s and 60s initial sl- became lh-, as opposed to (Old) Noldorin of the 1930s where it became thl-.