1640s, "on the other side of," from Latin ulterior "more distant, more remote, farther, on the farther side," comparative of *ulter "beyond" (from suffixed form of PIE root *al- "beyond"). The sense "not at present in view or consideration" (as in ulterior motives) is attested from 1735.
Middle English fer, from Old English feor "to a great distance, long ago," from Proto-Germanic *ferro (source also of Old Saxon fer, Old Frisian fir, Old Norse fiarre, Old High German fer, Gothic fairra), from PIE root *per- (1), base of words for "through, forward," with extended senses such as "across, beyond" (source also of Sanskrit parah "farther, remote, ulterior," Hittite para "outside of," Greek pera "across, beyond," Latin per "through," Old Irish ire "farther"). For vowel change, see dark (adj.). Paired with wide to mean "everywhere" since 9c.
1590s, from Greek, "daughters of the Hesperus," name given to the nymphs (variously numbered but originally three) who tended the garden with the golden apples. Their name has been mistakenly transferred to the garden itself.
The Gardens of the Hesperides with the golden apples were believed to exist in some island in the ocean, or, as it was sometimes thought, in the islands on the north or west coast of Africa. They were far-famed in antiquity; for it was there that springs of nectar flowed by the couch of Zeus, and there that the earth displayed the rarest blessings of the gods; it was another Eden. As knowledge increased with regard to western lands, it became necessary to move this paradise farther and farther out into the Western Ocean. [Alexander Murray, "Manual of Mythology," 1888]
Related: Hesperidean; Hesperidian.
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]