Do you know ghouls from goblins and ghosts?
c.1300, variant of further (q.v.), by 17c. it replaced ferrer as comparative of the descendant of Old English fierr "far" (itself a comparative but no longer felt as one). Vowel change influenced by the root vowel, and confusion with Middle English ferþeren "to assist, promote, advance" (see forth). There is no historical basis for the notion that farther is of physical distance and further of degree or quality.
Old English feorr "far, remote, distant, to a great distance, long ago," from Proto-Germanic *ferro (cf. Old Saxon ferr, Old Frisian fer, Old Norse fjarre, Dutch ver, Old High German ferro, German fern, Gothic fairra), from PIE *per- "through, across, beyond" (cf. Sanskrit parah "farther, remote, ulterior," Hittite para "outside of," Greek pera "across, beyond," Latin per "through," Old Irish ire "farther"). Far East "China, Japan, and surrounding regions" is from 1838.