The Doriathrin word for the “Moon” developed from the primitive name ᴹ✶Ithil (Ety/I²), though it is unclear how the [st] developed. Perhap it was of the same origin as istil “silver light, starlight” (Ety/SIL).
istilnoun. silver light, starlight
A Doriathrin noun for “silver light, starlight”, also given as istel, which developed from the root ᴹ√SIL (Ety/SIL). Tolkien said that it was probably originally a (ancient?) Quenya word learned from Melian. This apparently this means its development was unusual. Along with the identical Istil “Moon”, it is the only Ilkorin noun with a medial [st]. Helge Fauskanger suggested it must have developed by a strengthening of [s] to [st]. It may also be another example of a development from syllabic [ṣ], though elsewhere initial [s] became [es] before voiceless stops.