'Rainbow' is transparently 'rain-bow' — Old English 'regnboga.' The compound is shared across all Germanic.
An arc of colors visible in the sky when sunlight is refracted and dispersed by water droplets in the atmosphere, typically showing red, orange, yellow, green, blue, indigo, and violet.
From Old English "regnboga," a compound of "regn" ("rain") and "boga" ("bow, arch, anything curved"), both from Proto-Germanic: *regnaz ("rain") from PIE *h₃reǵ- ("to make wet, to rain"), and *bugô ("bow, bend") from PIE *bʰewgʰ- ("to bend"). The rain element produced Old Norse "regn," German "Regen," Dutch "regen," and Gothic "rign," while the PIE root *h₃reǵ- connects to Latin "rigāre" ("to water, to irrigate," yielding "irrigate"). The bow element gave Old Norse "bogi" (yielding English "bow" as a weapon and as a curve), German "Bogen" ("arch, bow"), and Dutch "boog." The compound "rain-bow" is a pan
Almost every Germanic language uses the same compound for rainbow: German 'Regenbogen,' Dutch 'regenboog,' Swedish 'regnbåge,' Norwegian 'regnbue,' Danish 'regnbue.' The compound is so old and so consistent that it almost certainly existed in Proto-Germanic as *regnabugô — making 'rainbow' one of the few complex compound words that can be confidently reconstructed for the proto-language.