Old English furðor, forðor "to a more advanced position, forward, onward, beyond, more distant; farther away; later, afterward; to a greater degree or extent, in addition; moreover," etymologically representing either "forth-er" or "fore-ther." The former would be from furðum (see forth) + comparative suffix *-eron-, *-uron- (compare inner, outer).
Alternative etymology (Watkins) traces it to Proto-Germanic *furthera-, from PIE *pr-tero- (source also of Greek proteros "former"), representing the root *per- (1) "forward" + comparative suffix also found in after, other. Senses of "in addition, to a greater extent" are later metaphoric developments.
It replaced or absorbed farrer, ferrer as comparative of far (itself a comparative but no longer felt as one). Farrer itself displaced Old English fierr in this job; farrer survived until 17c., then was reduced to dialect by rival farther. "The primary sense of further, farther is 'more forward, more onward'; but this sense is practically coincident with that of the comparative degree of far, where the latter word refers to real or attributed motion in some particular direction." [OED]