A noun for “rainbow” in the name Cirith Ninniach “Rainbow Cleft” (S/238). Ninniach “rainbow” is perhaps a combination of the S. nîn “watery” and S. iach “ford”, perhaps a metaphorical ford of water across the sky. The long nn in the initial element ninn- is tricky to explain, but might be an example of an exchange of a long vowel īn with a long consonant inn; hat-tip to Elaran for this suggestion.
Conceptual Development: The name Cirith Ninniach (and hence the word for “rainbow”) went through quite a few changes. The earliest iteration of the name was G. Cris a Teld Quing Ilon “Gully of the Rainbow Roof” (PE15/21) so that quing ilon “✱bow of heaven” was “rainbow”, but this was quickly revised to G. Cris Ilbranteloth which was the form used in the early narratives (LT2/150, 202). G. ilbrant “rainbow” appeared in the Gnomish Lexicon of the 1910s with variant ilvrant (GL/50). By popular etymology, this was connected to G. brant “bow”, but that was not correct (GL/24). The second element was actually G. rantha “bridge” (GL/65), and the b came from its initial element G. ilbar “heaven[s]” (GL/50), so it literally meant “✱heaven-bridge”.
In Silmarillion drafts from the 1930s, the name of the pass was changed to N. Cris-Ilfing >> Cirith Helvin “Rainbow Cleft” (SM/141, 146). Both ilfing and helvin have unclear etymologies, but they are probably early iterations of N. eilian(w) “rainbow, (lit.) sky-bridge” from The Etymologies of the 1930s, a combination of ᴹ√ƷEL “sky” with N. ianw “bridge” (Ety/ƷEL, YAT), with variant elianw (EtyAC/YAT). Cirith Ninniach “Rainbow Cleft” emerged in Silmarillion revisions from the 1950s-60s (WJ/256, 299).
Neo-Sindarin: Noldorin eilian(w) “sky-bridge” might be adapted into Neo-Sindarin as ᴺS. eiliant using later S. iant for “bridge”, as suggested in HSD (HSD), but I see no reason not to just use attested S. ninniach for “rainbow”.
See ianu and iant for a discussion