Etymology
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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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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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certain (adj.)

c. 1300, "determined, fixed," from Old French certain "reliable, sure, assured" (12c.), from Vulgar Latin *certanus, extended form of Latin certus "determined, resolved, fixed, settled," of things whose qualities are invariable, "established," also "placed beyond doubt, sure, true, proved; unerring, to be depended upon" (also source of Old French cert, Italian certo, Spanish cierto), originally a variant past participle of cernere "to distinguish, decide," literally "to sift, separate." This Latin verb comes from the PIE root *krei- "to sieve," thus "discriminate, distinguish," which is also the source of Greek krisis "turning point, judgment, result of a trial" (compare crisis).

The transferred sense, in reference to persons, "full of confidence in one's knowledge or judgment, made certain in reference to a matter or thing," is from mid-14c. (it also was a sense in Latin). The meaning "established as true beyond doubt" in English is from c. 1400. The meaning "indefinite, not specifically named, known but not described" is from late 14c.

Different as this seems to be from sense I, it is hardly separable from it in a large number of examples: thus, in [a certain hour], the hour was quite 'certain' or 'fixed', but it is not communicated to the reader; to him it remains, so far as his knowledge is concerned, quite indefinite; it may have been, as far as he knows, at any hour; though, as a fact, it was at a particular hour. [OED]

Lewis & Short write that Latin certus also was sometimes indefinite, "of things, the certainty of whose existence is given, but whose nature is not more definitely designated, or comes not into consideration ...."

Hence the euphemistic use, attested from mid-18c., as in woman of a certain age "an old maid;" woman of a certain description "disreputable woman;" in a certain condition "pregnant;" a certain disease "venereal disease;" of a certain weight "obese." Used with proper names from 1785, "often conveying a slight shade of disdain" [OED]. Certainer, certainest were common to c. 1750, but have fallen from proper use for some reason. Expression for certain "assuredly" is attested by early 14c.

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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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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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good-for-nothing (adj.)

"worthless," 1711, from adjectival phrase (see good (adj.)).

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summons (n.)

"authoritative call to be at a certain place for a certain purpose," late 13c., from Old French sumunse, noun use of fem. past participle of somondre (see summon (v.)). As a verb from 1650s.

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fag (v.2)

"put to work at certain duties, compel to work for one's benefit," 1806, from British public school slang fag (n.) "junior student who does certain duties for a senior" (1785), from fag (v.1). Related: Fagdom (1902); faggery "fatiguing labor" (1853). Brain-fag (1850) was an old term for "mental fatigue."

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regard (v.)

mid-14c., regarden, "consider" (that something is so or a certain way), from Old French regarder "to look at, take notice of," from re-, here perhaps an intensive prefix, + garder "look, heed," from a Germanic language (see guard (n.)).

Sense of "consider of importance or interest" is from 1510s. Meaning "look upon, observe" is from 1520s, as is that of "observe a certain respect toward." From 1610s as "look upon" (with a certain feeling), "have or show a certain feeling for." Related: Regarded; regarding.

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