Etymology
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behind (adv., prep.)

Old English behindan "at the back of, after," from bi "by" (see by) + hindan "from behind" (see hind (adj.)). The prepositional sense emerged in Old English. The figurative sense of "not so far advanced, not on equality with" is from c. 1200. The euphemistic noun meaning "backside of a person" is from 1786.

To do something behind (someone's) back "clandestinely" is from late 14c. Phrase behind the times is by 1826. Behind the scenes (1711) is from the theater; figurative sense attested by 1779.

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act (v.)

mid-15c., acten, "to act upon or adjudicate" a legal case, from Latin actus, past participle of agere "to set in motion, drive, drive forward," hence "to do, perform," also "act on stage, play the part of; plead a cause at law" (from PIE root *ag- "to drive, draw out or forth, move").

The verb is original in Latin, but most of the modern verbal senses in English probably are from the noun. The general sense of "to do, perform, transact" is from c. 1600. Of things, "do something, exert energy or force," by 1751. In theater use from 1590s as "perform as an actor" (intransitive), 1610s as "represent by performance on the stage" (transitive). The general meaning "perform specific duties or functions," often on a temporary basis, is by 1804.

To act on "exert influence on" is from 1810. To act up "be unruly" is by 1900 (in reference to a horse). Earlier it meant "acting in accordance with" a duty, expectation, or belief (1640s). To act out "behave anti-socially" (1974) is from psychiatric sense of "expressing one's unconscious impulses or desires" (acting out is from 1945). Related: Acted; acting.

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child (n.)

Old English cild "fetus, infant, unborn or newly born person," from Proto-Germanic *kiltham (source also of Gothic kilþei "womb," inkilþo "pregnant;" Danish kuld "children of the same marriage;" Old Swedish kulder "litter;" Old English cildhama "womb," lit. "child-home"); no certain cognates outside Germanic. "App[arently] originally always used in relation to the mother as the 'fruit of the womb'" [Buck]. Also in late Old English, "a youth of gentle birth" (archaic, usually written childe). In 16c.-17c. especially "girl child."

The wider sense "young person before the onset of puberty" developed in late Old English. Phrase with child "pregnant" (late 12c.) retains the original sense. The sense extension from "infant" to "child" also is found in French enfant, Latin infans. Meaning "one's own child; offspring of parents" is from late 12c. (the Old English word was bearn; see bairn). Figurative use from late 14c. Most Indo-European languages use the same word for "a child" and "one's child," though there are exceptions (such as Latin liberi/pueri).

The difficulty with the plural began in Old English, where the nominative plural was at first cild, identical with the singular, then c.975 a plural form cildru (genitive cildra) arose, probably for clarity's sake, only to be re-pluraled late 12c. as children, which is thus a double plural. Middle English plural cildre survives in Lancashire dialect childer and in Childermas.

Child abuse is attested by 1963; child-molester from 1950. Child care is from 1915. Child's play, figurative of something easy, is in Chaucer (late 14c.):

I warne yow wel, it is no childes pley To take a wyf withouten auysement. ["Merchant's Tale"]
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left (adj.)

c. 1200, "opposite of right," probably from Kentish and northern English forms of Old English *lyft "weak; foolish" (in lyft-adl "lameness, paralysis"). Compare East Frisian luf, Dutch dialectal loof "weak, worthless").

Sense of "opposite of right" is from the left being usually the weaker hand), a derived sense also found in cognate Middle Dutch and Low German luchter, luft. Compare Lithuanian kairys "left" and Lettish kreilis "left hand" both from a root that yields words for "twisted, crooked."

The usual Old English winstre/winestra "left" (adj.); "left hand," literally "friendlier," a euphemism used superstitiously to avoid invoking the unlucky forces connected with the left side (compare sinister). The Kentish word itself might have been originally a taboo replacement, if instead it represents PIE *laiwo- "considered conspicuous" (represented in Greek laios, Latin laevus, and Russian levyi). Greek also uses a euphemism for "left," aristeros "the better one" (compare also Avestan vairyastara- "to the left," from vairya- "desirable").

Meaning "being on the left-hand side" is from c. 1300. As an adverb from early 14c. For political senses, see left (n.). Used at least since c. 1600 in various senses of "irregular, illicit;" earlier proverbial sense was "opposite of what is expressed" (mid-15c.), for example over the left (shoulder) "not at all," added to a statement to negate or neglect what was just said (1705). To have two left feet "be clumsy" is attested by 1902.

Phrase out in left field "out of touch with pertinent realities" is attested from 1944, from the baseball fielding position that tends to be far removed from the play (left field in baseball attested by 1867; the fielding positions are from the point of view of the batter). The Parisian Left Bank (of the River Seine) has been associated with intellectual and artistic culture at least since 1893; Left Coast "Pacific Coast of the U.S." is by 1980s.

German link, Dutch linker "left" are said to be not directly related to these, being instead from Old High German slinc and Middle Dutch slink "left," related to Swedish linka "limp," slinka "dangle," and Old English slincan "crawl" (Modern English slink).

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left 

"remaining," past tense and past participle of leave (v.).

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left (n.)

c. 1200, "the left-hand side, the side opposite the right," from left (adj.). In military formations with reference to the center; of river banks it implies going in the direction the current flows; in an assembly in reference to the seat of the presiding officer; in baseball in reference to the point of view of the batter.

Political sense "the democratic or liberal party" arose from the custom of assigning those members of a legislative body to the left side of a chamber. This usage is first attested in English in 1837 (by Carlyle, in reference to the French Revolution), and probably is a loan-translation of French la gauche (1791), said to have originated during the seating of the French National Assembly in 1789 in which the nobility took the seats on the President's right and left the Third Estate to sit on the left. The term became general in U.S. and British political speech c. 1900. Century Dictionary and OED 2nd ed. both refer to this as primarily in reference to continental European politics.

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act (n.)

late 14c., "a thing done," from Latin actus "a doing; a driving, impulse, a setting in motion; a part in a play," and actum "a thing done" (originally a legal term), both from agere "to set in motion, drive, drive forward," hence "to do, perform," figuratively "incite to action; keep in movement, stir up" (from PIE root *ag- "to drive, draw out or forth, move").

The verb agere had a broad range of meaning in Latin, including "act on stage, play the part of; plead a cause at law; chase; carry off, steal." The theatrical ("part of a play," 1510s) and legislative (early 15c.) senses of the noun also were in Latin.

The meaning "one of a series of performances in a variety show" is from 1890. The meaning "display of exaggerated behavior" is from 1928, extended from the theatrical sense. In the act "in the process" is from 1590s, perhaps originally from a late 16c. sense of the act as "sexual intercourse." Act of God "uncontrollable natural force" is recorded by 1726.

An act of God is an accident which arises from a cause which operates without interference or aid from man (1 Pars. on Cont. 635); the loss arising wherefrom cannot be guarded against by the ordinary exertions of human skill and prudence so as to prevent its effect. [William Wait, "General Principles of the Law," Albany, 1879]

To get into the act "participate" is from 1947; to get (one's) act together "organize one's (disorderly) life" is by 1976, perhaps euphemistic.

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no-no (n.)

"an action, etc., that is forbidden or unacceptable," 1942, from a childish reduplication of no.

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No. 

as an abbreviation meaning (and pronounced) "number," 1660s, from Latin numero, ablative singular of numerus (see number (n.)).

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no (adv.)

"not in any degree, not at all," Middle English, from Old English na, from ne "not, no" + a "ever." The first element is from Proto-Germanic *ne (source also of Old Norse, Old Frisian, Old High German ne, Gothic ni "not"), from PIE root *ne- "not." Second element is from Proto-Germanic *aiwi-, extended form of PIE root *aiw- "vital force, life, long life, eternity." Ultimately identical to nay, and the differences of use are accidental.

As an adjective, "not any, not one, none" (c. 1200) it is reduced from Old English nan (see none), the final -n omitted first before consonants and then altogether. As an interjection making a negative reply to a statement or question, "not so," early 13c., from the adverb. As a noun, 1580s as "a denial; a negative vote," 1650s as "person who casts a negative vote."

Construction no X, no Y is attested from 1530s (in no peny no pardon). No problem as an interjection of assurance is attested by 1963. No way as a colloquial expression meaning "it can't be done" is attested by 1968 (noway (adv.) "not at all, in no respect, by no means" is from c. 1300). No-knock (adj.) in reference to police raids without permission or warning is by 1970, American English. Phrase no can do "it is not possible" is attested from 1827, a locution of English-speaking Chinese noted 19c. in China, Australia, and the West Coast of the United States.  

We repeated our advice again and again, but got no answer but a loud horse-laugh, and their national maxim of No can do: Europe fashion no do in China. ["Reminiscences of a Voyage to and from China," in Paxton's Horticultural Register, London, 1836]
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