Origin and history of for-

prefix usually meaning "away, opposite, completely," from Old English for-, indicating loss or destruction, but in other cases completion, and used as well with intensive or pejorative force, from Proto-Germanic *fur "before, in" (source also of Old Norse for-, Swedish för-, Dutch ver-, Old High German fir-, German ver-); from PIE *pr-, from root *per- (1) "forward," hence "in front of, before, toward, near, against." Ultimately from the same root as fore (adv.), and compare ver-.

In verbs the prefix denotes (a) intensive or completive action or process, or (b) action that miscarries, turns out for the worse, results in failure, or produces adverse or opposite results. In many verbs the prefix exhibits both meanings, and the verbs frequently have secondary and figurative meanings or are synonymous with the simplex. [ Middle English Compendium]

Probably originally in Germanic with a sense of "forward, forth," but it spun out complex sense developments in the historical languages. It is disused as a word-forming element in Modern English.

From its use in participles it came to be an intensive prefix of adjectives in Middle English (for example Chaucer's forblak "exceedingly black"), but all these now seem to be obsolete.

It is grievous to think how much less careful the English have been to preserve than to acquire. Why have we lost, or all but lost, the ver or for as a prefix,— fordone, forwearied, &c.; and the zer or to,— zerreissen, to rend, &c. Jugend, Jüngling : youth, youngling ; why is that last word now lost to common use, and confined to sheep and other animals? [Coleridge, "German Language," in "Omniana"]