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.
From late Old English as "in favor of." For and fore differentiated gradually in Middle English. For alone as a conjunction, "because, since, for the reason that; in order that" is from late Old English, probably a shortening of common Old English phrases such as for þon þy "therefore," literally "for the (reason) that."
Old English locian "use the eyes for seeing, gaze, look, behold, spy," from West Germanic *lokjan (source also of Old Saxon lokon "see, look, spy," Middle Dutch loeken "to look," Old High German luogen, German dialectal lugen "to look out"), a word of unknown origin. Breton lagud "eye" has been suggested as a possible cognate.
In Old English, usually with on; the use of at began 14c. As a word to call attention, c. 1200 (look out! "take notice" is from mid-15c.). Meaning "seek, search out" is c. 1300; meaning "to have a certain appearance, express or manifest by looks" is from c. 1400. Of objects, "to face in a certain direction," late 14c. To look like "have the appearance of" is from mid-15c. Look after "take care of" is from late 14c., earlier "to seek" (c. 1300), "to look toward" (c. 1200). Look into "investigate" is from 1580s. To look forward "anticipate" is c. 1600; especially "anticipate with pleasure" from mid-19c. To look over "scrutinize" is from mid-15c.
Look up is from c. 1200 in literal sense "raise the eyes;" as "research in books or papers" from 1690s. To look up to "regard with respect and veneration" is from 1719. To look down upon in the figurative sense "regard as beneath one" is from 1711; to look down one's nose is from 1921. To not look back "make no pauses" is colloquial, first attested 1893. In look sharp (1711), sharp originally was an adverb, "sharply." To look around "search about, look round" is from 1883.
