Etymology
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for (prep.)
Old English for "before, in the sight of, in the presence of; as far as; during, before; on account of, for the sake of; in place of, instead of," from Proto-Germanic *fur "before; in" (source also of Old Saxon furi "before," Old Frisian for, Middle Dutch vore, Dutch voor "for, before;" German für "for;" Danish for "for," før "before;" Gothic faur "for," faura "before"), from PIE root *per- (1) "forward," hence "in front of, before," etc.

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."
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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."

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.

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go for (v.)
1550s, "be taken or regarded as," also "be in favor of," from go (v.) + for (adv.). Meaning "attack, assail" is from 1880. Go for broke is from 1951, American English colloquial.
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good-for-nothing (adj.)
"worthless," 1711, from adjectival phrase (see good (adj.)).
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tit for tat 
1550s, possibly an alteration of tip for tap "blow for blow," from tip (v.3) "tap" + tap "touch lightly." Perhaps influenced by tit (n.2).
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free-for-all (n.)
"mass brawl" (one in which all may participate), 1918, from earlier adjective use (1868), especially in reference to open horse races, American English. Earlier as a noun in reference to free-for-all horse and motorcar races.
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forever (adv.)
late 14c., for ever; from for + ever. Often written as one word from late 17c. As a noun by 1858. Emphatic forevermore is from 1819.
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forfend (v.)
also forefend, late 14c., "to protect; to prohibit; to avert, fend off, prevent," a hybrid from for- + fend "to ward off." Archaic, if not obsolete.
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forsooth (adv.)
Old English forsoð "indeed, in truth, verily," from for-, perhaps here with intensive force (or else the whole might be "for a truth"), + soð "truth" (see sooth). Regarded as affected in speech by c. 1600.
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forswear (v.)
Old English forswerian "swear falsely" (intransitive), also "abandon or renounce on oath" (transitive), from for- "completely" + swerian "to swear" (see swear (v.)). Related: Forswore; forsworn; forswearing.
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