forsooth (adv.)

"truth, reality, fact," Old English soð "truth, justice, righteousness, rectitude; reality, a true situation, certainty," noun use of soð (adj.) "true, genuine, real; just, righteous," originally *sonð-, from Proto-Germanic *santhaz (source also of Old Norse sannr, Old Saxon soth, Old High German sand "true," Gothic sunja "truth"). Compare forsooth.

The group is related to Old English synn "sin" and Latin sontis "guilty" (truth is related to guilt via "being the one;" see sin (v.)), from PIE *hes-ont- "being, existence," thus "real, true" (from present participle of root *es- "to be"), also preserved in Latin sunt "they are" and German sind.

Archaic in English, it is the root of modern words for "true" in Swedish (sann) and Danish (sand). It was in common use until mid-17c. then obsolete until revived as an archaism early 19c. by Scott, etc. It was used for Latin pro- in translating compounds into Old English, such as soðtacen "prodigy," soðfylgan "prosequi."

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"]