further (adv., adj.) Look up further at Dictionary.com
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- (see also inner, outer).

Alternative etymology traces it to Proto-Germanic *furþeron-, from PIE *pr-tero (source also of 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.
further (v.) Look up further at Dictionary.com
Old English (ge)fyrðan "further, impel;" see further (adj.). Compare Middle Low German vorderen, Old High German furdiran, German fördern. Related: Furthered; furthering.