Rainbow

google

ref

Old English regnboga (see rain, bow1).

wiktionary

ref

From Middle English reinbowe, reinboȝe, from Old English reġnboga(“rainbow”), from Proto-Germanic *regnabugô(“rainbow”), equivalent to rain +‎ bow(“arch”). Cognate with West Frisian reinbôge(“rainbow”), Dutch regenboog(“rainbow”), German Regenbogen(“rainbow”), Danish regnbue(“rainbow”), Swedish regnbåge(“rainbow”), Icelandic regnbogi(“rainbow”).

etymonline

ref

rainbow (n.)

"arc of prismatic colors formed by the refraction of light rays by drops of rain or vapor," Middle English rein-bowe, from Old English renboga; see rain (n.) + bow (n.). Common Germanic compound (Old Frisian reinboga, Old Norse regnbogi, Swedish regenbåge, Dutch regenboog, German Regenbogen). The American rainbow trout (1876) is so called for its resplendent colors. Old English also had scurboga "shower-bow."