Origin and history of forth

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.

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.