Old English furðor (adv.), furðra (adj.), etymologically representing either "forth-er" or "fore-ther." The former would be from furðum (see forth) + comparative suffix *-eron-, *-uron- (cf. inner, outer).
Alternative etymology traces it to Proto-Germanic *furþeron-, from PIE *pr-tero, (cf. Greek proteros "former"), from root of fore + comparative suffix also found in after, other. Senses of "in addition, to a greater extent" are later metaphoric developments.