forswear (v.)

from Old English forsworen, "perjured," past participle of forswerian "to swear falsely" (see forswear).

Middle English sweren, from Old English swerian, swerigean, "take or utter an oath, make a solemn declaration with an appeal to divinity" (class VI strong verb; past tense swor, past participle sworen), from Proto-Germanic *swērjanan (source also of Old Saxon swerian, Old Frisian swera, Old Norse sverja, Danish sverge, Middle Dutch swaren, Old High German swerien, German schwören, Gothic swaren "to swear").

This is of uncertain origin. The old explanation (Pokorny, Watkins) has it from a PIE *swer- "to speak, talk, say" (source also of Old Church Slavonic svara "quarrel," Oscan sverrunei "to the speaker"). Boutkan suspects a substratum word, or, if it is IE, writes that a connection to Latin verbum "seems more promising."

It is related to the second element in answer. A Middle English noun sware meant "an answer, a reply; speech, utterance," from Old English -swaru, and from the Old Norse cognate.

The secondary sense of "use profane language" (early 15c.) probably developed from the notion of "invoke sacred names profanely or blasphemously" (mid-14c.).

[Swearing and cursing] are entirely different things : the first is invoking the witness of a Spirit to an assertion you wish to make ; the second is invoking the assistance of a Spirit, in a mischief you wish to inflict. When ill-educated and ill-tempered people clamorously confuse the two invocations, they are not, in reality, either cursing or swearing ; but merely vomiting empty words indecently. True swearing and cursing must always be distinct and solemn .... [Ruskin, "Fors Clavigera"]

To swear off "desist, abjure, renounce solemnly, as with a vow" is by 1839. To swear in "install (someone) in office by administration of an oath" is attested from 1700 in modern use, echoing Middle English, where to be sworn was to be admitted to office by formal oath (c. 1200).

To swear by is from early 13c., originally in reference to a divine being or sacred object; the colloquial sense of "treat as an infallible authority, place great confidence in" is by 1815.

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"]