In Tolkien’s later writings this root primarily meant “fade”, but its earliest precursor ᴱ√QELE from the Qenya Lexicon of the 1910s was glossed “perish, die, decay, fade”, with derivatives like ᴱQ. qele- of the same meaning, ᴱQ. qelet “corpse”, and ᴱQ. qelme “ruin, utter end, perdition, end, death” (GL/76). Derivatives from the contemporaneous Gnomish Lexicon had a similar semantic scope, such as G. cwel- “fade, wither”, G. cweleg “corpse”, and G. cweloth “fading”, most notably as an element in G. lasgweloth “leaf-fading, autumn” (GL/28); the connection between this root and words for “autumn” survived in Tolkien’s later conception of the languages.
In The Etymologies of the 1930s, the meaning of the root ᴹ√KWEL seems to have softened somewhat to “fade (away), wither” with more of a sense of waning rather than outright death, though ᴹQ. qelet “corpse” remained among its derivatives (Ety/KEL, KWEL). In the 1930s it still was related to words for “autumn”, notably N. lhasbelin (Ety/LAS¹) “leaf-fall, autumn” and ᴹQ. Narqelion “Fire-fading, Autumn” (Ety/NAR¹). The latter seems to have been modified to Q. Narquelië (and S. Narbeleth) as the Elvish word for October, literally “Sun-fading”, in the Lord of the Ring appendixes (LotR/1107). The root was also the basis for Q. quellë “autumn” (LotR/1111), though its Sindarin equivalent (firith) was based on a different root.
Starting in the 1930s Tolkien indicated this root had an etymological relationship with √KEL “flow (down)”; see that entry for details.