An adjective in The Etymologies of the 1930s glossed “naked” and derived from primitive ᴹ✶skelnā under the root ᴹ√SKEL (Ety/SKEL). This word was originally hall “naked” under an earlier but deleted form of the root ᴹ√SKAL (EtyAC/SKEL). The root ᴹ√SKEL was also the basis for the verb N. heltha- “to strip”, and its Quenya derivative ᴹQ. helda was at one point was glossed “stripped bare” (Ety/SKAL¹), so the word hell seems to mean “naked” in the sense “✱stripped (of clothing or other covering)”.
Conceptual Development: There are a couple of earlier “naked” words in the Gnomish Lexicon of the 1910s that seems to be similarly derived. G. dautha “naked, stripped” was related to daf- “to strip, flay” (GL/29) and G. hulc “naked” (GL/49) is like the cognate of ᴱQ. hulqa “naked” under the early root ᴱ√HULU “strip” (QL/41). In later writings, some “naked” words were derived from √PAR “peel” instead (PE17/86, 171).
A noun in The Etymologies of the 1930s glossed “frost” derived from the root ᴹ√KHEL “freeze”, but Tolkien deleted this short root and its derivatives, keeping only the longer form ᴹ√KHELEK (Ety/KHEL).
Conceptual Development: The Gnomish Lexicon of the 1910s had a number of similar “frost” related words: G. heloth “frost”, G. hîl {“ice-cold, icy” >>} “frost” as well as G. helfileg “frost on panes, etc.” (GL/48-49), all based on the early root ᴱ√HELE “freeze”, the last with the added element G. fileg “fern”.
Neo-Sindarin: Tolkien probably deleted N. hell “frost” because he abandoned the short root ᴹ√KHEL, but short √KHEL did reappear in later writings (PE17/116), so it is tempting to restore hell “frost” as well. However, this conflicts with other attested words like N. hell “naked”, so I think it is better to use a neologism like ᴺS. nich for “frost”; see that entry for discussion.