Old English abufan, earlier onbufan, from on (see on; also see a-(1)) + bufan "over," compound of be "by" (see by) + ufan "over/high," from Proto-Germanic *ufan- (cognates: Old Saxon, Old High German oban, German oben), from PIE root *upo (see up, adv.). Meaning "in addition" first corded 1590s.