forsake v.

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."

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. Disused as a word-forming element in Modern English. Ultimately from the same root as fore (adv.). 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.

[purpose], Middle English sake "strife, discord, enmity, dispute; legal dispute; blame, sin, guilt;" from Old English sacu "a cause at law, crime, dispute, guilt," from Proto-Germanic *sako "affair, thing, charge, accusation" (source also of Old Norse sök "charge, lawsuit, effect, cause," Old Frisian seke "strife, dispute, matter, thing," Dutch zaak "lawsuit, cause, sake, thing," German Sache "thing, matter, affair, cause"), from PIE root *sag- "to investigate, seek out" (source also of Old English secan, Gothic sokjan "to seek;" see seek).

Much of the word's original meaning has been taken over by case (n.1) and cause (n.), and it survives largely in phrases for the sake of and for _______'s sake "out of consideration or regard for" a person or thing (c. 1200, as for God's sake, early 14c.), both those formations are said to be probably from Norse, as their like has not been found in Old English.