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 Hiswelókë’s Sindarin Dictionary (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”.