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 HSD (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-.