Etymology
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it (pron.)

Old English hit, neuter nominative and accusative of third person singular pronoun, from Proto-Germanic demonstrative base *khi- (source also of Old Frisian hit, Dutch het, Gothic hita "it"), from PIE *ko- "this" (see he). Used in place of any neuter noun, hence, as gender faded in Middle English, it took on the meaning "thing or animal spoken about before."

The h- was lost due to being in an unemphasized position, as in modern speech the h- in "give it to him," "ask her," is heard only "in the careful speech of the partially educated" [Weekley]. It "the sex act" is from 1610s; meaning "sex appeal (especially in a woman)" first attested 1904 in works of Rudyard Kipling, popularized 1927 as title of a book by Elinor Glyn, and by application of It Girl to silent-film star Clara Bow (1905-1965). In children's games, the meaning "the one who must tag or catch the others" is attested from 1842.

From Old English as nominative of an impersonal verb or statement when the thing for which it stands is implied (it rains, it pleases me). After an intransitive verb, used transitively for the action denoted, from 1540s (originally in fight it out). That's it "there is no more" is from 1966; this is it "the anticipated or dreaded moment has arrived" is from 1942.

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ask (v.)

Old English ascian "ask, call for an answer; make a request," from earlier ahsian, from Proto-Germanic *aiskojanan (source also of Old Saxon escon, Old Frisian askia "request, demand, ask," Middle Dutch eiscen, Dutch eisen "to ask, demand," Old High German eiscon "to ask (a question)," German heischen "to ask, demand"), from PIE *ais- "to wish, desire" (source also of Sanskrit icchati "seeks, desires," Armenian aic "investigation," Old Church Slavonic iskati "to seek," Lithuanian ieškau, ieškoti "to seek").

Form in English influenced by a Scandinavian cognate (such as Danish æske; the Old English would have evolved by normal sound changes into ash, esh, which was a Midlands and southwestern England dialect form). Modern dialectal ax is as old as Old English acsian and was an accepted literary variant until c. 1600. Related: Asked; asking.

Old English also had fregnan/frignan which carried more directly the sense of "question, inquire," and is from PIE root *prek-, the common source of words for "ask" in most Indo-European languages (see pray). If you ask me "in my opinion" is attested from 1910.

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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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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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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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know-it-all (n.)
"one deemed (over)full of information or correct answers," 1895, from verbal phrase; see know (v.). Earlier in the same sense was know-all (1862); and Mr. Know-All was a minor character in Bunyan's "The Holy War" (1682).
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go-it-alone (adj.)
attested by 1953 (in reference to U.S. foreign policy proposals), from an American English verbal phrase attested by 1842 and meaning "do anything without assistance." Go it as colloquial for "to act" (especially in a determined or vigorous way) is from 1825; hence also American English go it blind (1842) in reference to something done without regard for consequences.
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good-for-nothing (adj.)
"worthless," 1711, from adjectival phrase (see good (adj.)).
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do-it-yourself (adj.)

as a modifier, attested by 1941. The expression is much older (1610s). Related: Do-it-yourselfer.

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