means An arc of spectral colors that appears in the sky when sunlight is refracted and reflected through water droplets, usually after rain.

from A plain Old English compound: 'regn' (rain) plus 'boga' (bow, as in a bent arch or the kind you'd shoot arrows from). So it has always meant exactly what it says — a bow made of rain. The 'boga' part is the same ancient root that gives us elbow, the bough of a tree, and anything that bends, all tracing back to a Germanic word for bending.