An Etymological Dictionary of the German Language/Regen
Regen, m., from the equiv. MidHG. rëgen, OHG. rëgan, m., ‘rain’; common to Teut. in the same sense; comp. the corresponding Goth. rign, n., OIc. regn, n., AS. rëgn, m., E. rain, Du. regen, OSax. rëgan. Primit, Teut. regna-, from pre-Teut. *reghno-, probably represents *mreghno-, if the word be connected with Gr. βρέχειν (for μρεχ-, μβρεχ-), ‘to wet’; Lat. rĭgare, ‘to water, wet,’ may belong to the same Aryan root mregh, The compound Regenbogen is found in all the Teut. languages; MidHG. rëgenboge, OHG. rëganbogo (Du., however, waterboog), E. rainbow, from AS. rëgnboga, OIc. regnboge, Goth. *rìgnbuga.