verbal phrase, mid-13c. (intrans.), "express openly, present to view or consideration, make fully known;" c. 1400 as "leave, begin a journey" (set out in the same sense is from late 14c.); see set (v.) + forth (adv.). The notion of set involved in it is "proceed in a specified direction," hence begin to move" (attested from late Old English). From late 14c. as "prepare and send out, issue" (a commandment, etc.). Of a price from 1520s. Intransitive sense of "go, advance, begin to march" is from mid-14c.
late 15c., "about to happen or appear," present-participle adjective from Middle English forthcomen, from Old English forðcuman "to come forth, come to pass;" see forth + come (v.). Meaning "informative, responsive" is from 1835, via the notion of "in such a position or condition, as a person or a thing, that his or its presence when needed can be counted on." A once-common verb formation; English also had forthbring, forthcall, forthdo, forthgo, forthpass, forthset, all now obsolete.
Middle English aforth, from Old English geforðian "to put forth, contribute; further, advance; carry out, accomplish," from ge- completive prefix (which in Middle English regularly reduces to a-; see a- (1)) + forðian "to further," from forð "forward, onward" (see forth).
The prefix shift to af- took place 16c. under mistaken belief that it was a Latin word in ad-; change of -th- to -d- took place late 16c. (and also transformed burthen, spither, murther, etc. into their modern forms).
The notion of "accomplish" (late Old English) gradually became "be able to bear the expense of, have enough money" to do something (late 14c.), and the original senses became obsolete. Of things, "be capable of yielding," 1580s, which is the sense in afford (one) an opportunity. Related: Afforded; affording.
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]
Proto-Indo-European root forming prepositions, etc., meaning "forward," and, by extension, "in front of, before, first, chief, toward, near, against," etc.
It forms all or part of: afford; approach; appropriate; approve; approximate; barbican; before; deprive; expropriate; far; first; for; for-; fore; fore-; forefather; foremost; former (adj.); forth; frame; frau; fret; Freya; fro; froward; from; furnish; furniture; further; galore; hysteron-proteron; impervious; improbity; impromptu; improve; palfrey; par (prep.); para- (1) "alongside, beyond; altered; contrary; irregular, abnormal;" paradise; pardon; paramount; paramour; parvenu; pellucid; per; per-; percent; percussion; perennial; perestroika; perfect; perfidy; perform; perfume; perfunctory; perhaps; peri-; perish; perjury; permanent; permeate; permit; pernicious; perpendicular; perpetual; perplex; persecute; persevere; perspective; perspire; persuasion; pertain; peruse; pervade; pervert; pierce; portray; postprandial; prae-; Prakrit; pre-; premier; presbyter; Presbyterian; preterite; pride; priest; primal; primary; primate; primavera; prime; primeval; primitive; primo; primogenitor; primogeniture; primordial; primus; prince; principal; principle; prior; pristine; private; privilege; privy; pro (n.2) "a consideration or argument in favor;" pro-; probably; probe; probity; problem; proceed; proclaim; prodigal; produce; profane; profess; profile; profit; profound; profuse; project; promise; prompt; prone; proof; proper; property; propinquity; prophet; prose; prostate; prosthesis; protagonist; Protean; protect; protein; Proterozoic; protest; proto-; protocol; proton; protoplasm; Protozoa; proud; prove; proverb; provide; provoke; prow; prowess; proximate; Purana; purchase; purdah; reciprocal; rapprochement; reproach; reprove; veneer.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pari "around, about, through," parah "farther, remote, ulterior," pura "formerly, before," pra- "before, forward, forth;" Avestan pairi- "around," paro "before;" Hittite para "outside of," Greek peri "around, about, near, beyond," pera "across, beyond," paros "before," para "from beside, beyond," pro "before;" Latin pro "before, for, on behalf of, instead of," porro "forward," prae "before," per "through;" Old Church Slavonic pra-dedu "great-grandfather;" Russian pere- "through;" Lithuanian per "through;" Old Irish ire "farther," roar "enough;" Gothic faura "before," Old English fore (prep.) "before, in front of," (adv.) "before, previously," fram "forward, from," feor "to a great distance, long ago;" German vor "before, in front of;" Old Irish air- Gothic fair-, German ver-, Old English fer-, intensive prefixes.