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”.
This root and ones like it were tied to spider words for much of Tolkien’s life, most notably in the name S. Ungoliant and its precursors. The earliest iteration of this root was unglossed ᴱ√GUŊU in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. ungwe/G. gung “spider” (QL/98); Tolkien marked the root with a “?” and given that the Qenya forms had no initial consonant, the actual root may have been ✱ᴱ√ƷUŊU. In The Lost Tales of the 1910s, Tolkien changed G. Gungliont to G. Ungoliont (LT1/156), and in the contemporaneous Gnomish Lexicon the word for “spider” was G. ungwi (GL/75), so it seems Tolkien revised {✱ᴱ√ƷUŊU >>} ✱ᴱ√UŊU.
In The Etymologies of the 1930s, unglossed ᴹ√UÑG had derivatives like ᴹQ. ungwe “gloom” and ᴹQ. ungo “cloud, dark shadow” (Ety/UÑG), and it was the second element ᴹQ. liante in ᴹQ. Ungoliante that meant “spider” (Ety/SLIG). However, in notes from 1969 Tolkien gave ✱ungu- as the basis for “spider” words (PE22/160), as reflected in Q. ungwë “spider’s web” (LotR/1122) and S. ungol “spider” in his later writings (Let/180; RC/490, 767).