Etymology
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six (adj., n.)

"1 more than five, twice three; the number which is one more than five; a symbol representing this number;" Old English siex, six, sex, from Proto-Germanic *seks (source also of Old Saxon and Danish seks, Old Norse, Swedish, and Old Frisian sex, Middle Dutch sesse, Dutch zes, Old High German sehs, German sechs, Gothic saihs), from PIE *s(w)eks (source also of Sanskrit sas, Avestan kshvash, Persian shash, Greek hex, Latin sex, Old Church Slavonic sesti, Polish sześć, Russian shesti, Lithuanian šeši, Old Irish se, Welsh chwech).

Six-shooter, usually a revolver with six chambers, is first attested 1844; six-pack of beverage containers is from 1952, of abdominal muscles by 1995. Six of one and half-a-dozen of the other "little difference" is recorded from 1833. Six-figure in reference to hundreds of thousands (of dollars, etc.) is from 1840. Six feet under "dead" is from 1942.

Phrase at sixes and sevens originally was "hazarding all one's chances," first in Chaucer, perhaps from dicing (the original form was on six and seven); it could be a corruption of at cinque and sice "exposed to great risk" (1530s), literally "at five and six," using the French names (which were common in Middle English) for the highest numbers on the dice. Meaning "at odds, in disagreement or confusion" is from 1785, perhaps via a notion of "left unsettled."

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war (n.)

late Old English wyrre, werre "large-scale military conflict," from Old North French werre "war" (Old French guerre "difficulty, dispute; hostility; fight, combat, war;" Modern French guerre), from Frankish *werra, from Proto-Germanic *werz-a- (source also of Old Saxon werran, Old High German werran, German verwirren "to confuse, perplex"), from PIE *wers- (1) "to confuse, mix up". Cognates suggest the original sense was "to bring into confusion."

Spanish, Portuguese, and Italian guerra also are from Germanic; Romanic peoples turned to Germanic for a "war" word possibly to avoid Latin bellum (see bellicose) because its form tended to merge with bello- "beautiful." There was no common Germanic word for "war" at the dawn of historical times. Old English had many poetic words for "war" (wig, guð, heaðo, hild, all common in personal names), but the usual one to translate Latin bellum was gewin "struggle, strife" (related to win (v.)).

First record of war-time is late 14c. Warpath (1775) originally is in reference to North American Indians, as are war-whoop (1761), war-paint (1826), and war-dance (1757). War crime is attested from 1906 (in Oppenheim's "International Law"). War chest is attested from 1901; now usually figurative. War games translates German Kriegspiel (see kriegspiel).

The causes of war are always falsely represented ; its honour is dishonest and its glory meretricious, but the challenge to spiritual endurance, the intense sharpening of all the senses, the vitalising consciousness of common peril for a common end, remain to allure those boys and girls who have just reached the age when love and friendship and adventure call more persistently than at any later time. The glamour may be the mere delirium of fever, which as soon as war is over dies out and shows itself for the will-o'-the-wisp that it is, but while it lasts no emotion known to man seems as yet to have quite the compelling power of this enlarged vitality. [Vera Brittain, "Testament of Youth"]
The world will never have lasting peace so long as men reserve for war the finest human qualities. [John Foster Dulles, Speech on the Marshall Plan, 1948]
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day (n.)

Old English dæg "period during which the sun is above the horizon," also "lifetime, definite time of existence," from Proto-Germanic *dages- "day" (source also of Old Saxon, Middle Dutch, Dutch dag, Old Frisian di, dei, Old High German tag, German Tag, Old Norse dagr, Gothic dags), according to Watkins, from PIE root *agh- "a day."  He adds that the Germanic initial d- is "of obscure origin." But Boutkan says it is from PIE root *dhegh- "to burn" (see fever). Not considered to be related to Latin dies (which is from PIE root *dyeu- "to shine").

Meaning originally, in English, "the daylight hours;" it expanded to mean "the 24-hour period" in late Anglo-Saxon times. The day formerly began at sunset, hence Old English Wodnesniht was what we would call "Tuesday night." Names of the weekdays were not regularly capitalized in English until 17c.

From late 12c. as "a time period as distinguished from other time periods." Day-by-day "daily" is from late 14c.; all day "all the time" is from late 14c.  Day off "day away from work" is attested from 1883; day-tripper first recorded 1897. The days in nowadays, etc. is a relic of the Old English and Middle English use of the adverbial genitive.

All in a day's work "something unusual taken as routine" is by 1820. The nostalgic those were the days is attested by 1907. That'll be the day, expressing mild doubt following some boast or claim, is by 1941. To call it a day "stop working" is by 1919; earlier call it a half-day (1838). One of these days "at some day in the near future" is from late 15c. One of those days "a day of misfortune" is by 1936.

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war (v.)
"to make war on," mid-12c.; see war (n.). Related: Warred; warring.
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cold war (n.)

"nonhostile belligerency," used in print October 1945 by George Orwell; popularized in U.S. c. 1947 by U.S. statesman Bernard Baruch (1870-1965). Hence hot war (1947).

More than any other time in history, mankind faces a crossroads. One path leads to despair and utter hopelessness. The other, to total extinction. Let us pray we have the wisdom to choose correctly. [Woody Allen, from "My Speech to the Graduates," 1979]
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world war (n.)

attested by 1898 as a speculation.

If through fear of entangling alliances the United States should return the Philippines to Spain, Mr. Page asserted that the predatory nations would swoop down upon them and a world war would result. [New York Times, Dec. 16, 1898]

Applied to the first one almost as soon as it began in 1914 ("England has Thrown Lot with France in World War" — headline, Pittsburgh Press, Aug. 2, 1914). World War I was coined 1939, replacing Great War as the most common name for it; First World War, World War II, and Second World War all also are from 1939. Old English had woruldgewinn, woruldgefeoht, both of which might be translated "world war," but with "world" in the sense of "earthly, secular."

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War of 1812 
In reference to the conflict between the U.S. and Great Britain, so called in U.S. by 1815.
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deep six (n.)

"place where something is discarded," by 1921 (in phrase give (something) the deep six), originally in motorboating slang, perhaps from earlier underworld noun sense of "the grave" (1929), which is perhaps a reference to the usual grave depth of six feet. But the phrase (in common with mark twain) also figured in sailing jargon, of sounding, for a measure of six fathoms:

As the water deepened under her keel the boyish voice rang out from the chains: "By the mark five—and a quarter less six—by the deep six—and a half seven—by the deep eight—and a quarter eight." ["Learning the Road to Sea," in Outing magazine, February 1918]

In general use by 1940s. As a verb from 1953.

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eighty-six (v.)
slang for "eliminate," 1936, originated at lunch counters, a cook's word for "none" when asked for something not available, probably rhyming slang for nix.
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good-day (n.)
early 12c., "a fortunate day," also, generally, "good fortune;" from good (adj.) + day (n.). As a salutation in parting, haue godne day "have good day" is recorded from c. 1200; good day as a greeting is from late 14c.
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