From *lewk-(“to shine”) + *-sneh₂. Alternatively from *lewk-s- + *-néh₂, from the s-stem of the same root (compare Proto-Indo-Iranian*ráwčas(“day; light”)). In this case, originally a substantivized adjective *lewksnós or *lowksnós(“shiny, bright”) (compare Middle High Germanliehsen(“bright”) from *léwksnos, Ancient Greekλύχνος(lúkhnos, “lamp”) from *luksnos[1]), as the divergent meanings of Avestan, Persian and Old Prussian indicate, and the fact that Iranian has cognates that are adjectives, not nouns. The original Proto-Indo-European lexeme for "moon" was apparently *mḗh₁n̥s, and *lówksneh₂ may have originated as a later poetic synonym.
^ De Vaan, Michiel (2008) chapter 352, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, page lūna
^ Sihler, Andrew L. (1995) New Comparative Grammar of Greek and Latin, Oxford, New York: Oxford University Press, →ISBN, page 56: “*lewk-sneH₂-”
^ Derksen, Rick (2015) “lauxnos”, in Etymological Dictionary of the Baltic Inherited Lexicon (Leiden Indo-European Etymological Dictionary Series; 13), Leiden, Boston: Brill, →ISBN, page 561: “*louk-sneh₂”
↑ 4.04.1De Vaan, Michiel (2008) “lūna”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, page 354: “*louksnā-”
