A noun for “cobweb” in The Etymologies of the 1930s derived from primitive ᴹ✶slignē under the root ᴹ√SLIG (Ety/SLIG).
Conceptual Development: In the Qenya Lexicon of the 1910s Tolkien gave ᴱQ. līnẹ “thread, cotton” under the early root ᴱ√LI+ya (QL/53).
Neo-Quenya: This word is somewhat questionable, since in later writings, Tolkien derived spider words from √ungu-. However, I prefer to interpret ᴹ√(S)LIG as having to do with threads for purposes of Neo-Eldarin, and “cobweb” could be from a sense “collection of threads”. With this interpretation, I think it is also plausible to include the sense “cotton” from Early Qenya, though for “thread” I’d use other words like Q. ipsin or [ᴹQ.] lia. Helge Fauskanger suggested (QQ/líne) that this word should be ✱hlíne given primitive initial sl-, but there is another primitive form ᴹ✶ligā under the root, so perhaps the Quenya derivatives were (a) based from a variant root ᴹ√LIG or (b) reflect the Third Age Quenya pronunciation as voiced l (LotR/1114).
A word in in the Declension of Nouns of the early 1930s glossed {“fern” >>} “cobweb” (PE21/33 and note #3). Its initial gloss was probably a later iteration of ᴱQ. filqe “fern” from the Qenya Lexicon of the 1910s (QL/38), and in later writings filit (filik-) was a word for “small bird, sparrow” (Ety/PHILIK; PE21/56), so this was likely a transient form.