An Etymological Dictionary of the German Language/Annotated/Regen
Regen, masculine, from the equivalent Middle High German rëgen, Old High German rëgan, masculine, ‘rain’; common to Teutonic in the same sense; compare the corresponding Gothic rign, neuter, Old Icelandic regn, neuter, Anglo-Saxon rëgn, masculine, English rain, Dutch regen, Old Saxon rëgan. Primit, Teutonic regna-, from pre-Teutonic *reghno-, probably represents *mreghno-, if the word be connected with Greek βρέχειν (for μρεχ-, μβρεχ-), ‘to wet’; Latin rĭgare, ‘to water, wet,’ may belong to the same Aryan root mregh, The compound Regenbogen is found in all the Teutonic languages; Middle High German rëgenboge, Old High German rëganbogo (Dutch, however, waterboog), English rainbow, from Anglo-Saxon rëgnboga, Old Icelandic regnboge, Gothic *rìgnbuga.