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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gun (n.)

mid-14c., gunne "an engine of war that throws rocks, arrows or other missiles from a tube by the force of explosive powder or other substance," apparently a shortening of woman's name Gunilda, found in Middle English gonnilde "cannon" and in an Anglo-Latin reference to a specific gun from a 1330 munitions inventory of Windsor Castle ("... una magna balista de cornu quae Domina Gunilda ..."). Also compare gonnilde gnoste "spark or flame used to fire a cannon" (early 14c.).

The woman's name is from Old Norse Gunnhildr, a compound of gunnr and hildr, both meaning "war, battle." First element from PIE *gwhen- "to strike, kill" (see bane); for second, see Hilda.

The identification of women with powerful weapons is common historically (such as Big Bertha, Brown Bess, Mons Meg, etc.).

Or perhaps gun is directly from Old Norse gunnr "battle." The word was perhaps influenced by or confirmed by (or possibly from) Old French engon, dialectal variant of engin "engine."

Meaning grew with technology, from cannons to firearms as they developed 15c.; popularly applied to pistols and revolvers from 1744. In modern military use the word is restricted to cannons (which must be mounted), especially long ones used for high velocity and long trajectory. Hence great guns (1884 as an exclamation) distinguished from small guns (such as muskets) from c. 1400. Meaning "thief, rascal" is from 1858. For son of a gun, see son. To jump the gun (1912, American English) is a figurative use from track and field. Guns "a woman's breasts" (especially if prominent) attested by 2006.

[G]un covers firearms from the heaviest naval or siege guns (but in technical use excluding mortars and howitzers) to the soldier's rifle or the sportsman's shotgun, and in current U.S. use even the gangster's revolver. In the other European languages there is no such comprehensive word, but different terms for the small or hand gun of the soldier or sportsman (even these, sometimes differentiated) and the heavy naval guns or artillery pieces .... [Buck, 1949]
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gun (v.)
"shoot with a gun," 1620s, from gun (n.). Related: Gunned; gunning. The sense of "accelerate an engine" is from 1930, from earlier phrase give (her) the gun (1917), which appears to have originated in pilots' jargon in World War I; perhaps from the old military expression give a gun "order a gun to be fired" (c. 1600).
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machine-gun (n.)

"a gun which by means of a mechanism delivers a continuous fire of projectiles," 1870, from machine (n.) + gun (n.). As a verb, "to shoot down or kill by means of a machine gun," it is attested by 1915. Related: Machine-gunned; machine-gunning; machine-gunner.

A man is entitled to the fruits of his labor, and to assert a just claim is a duty as well as a right. In the year 1861 I first conceived the idea of a machine gun, which has been ever since the great controlling idea of my life; and it certainly cannot be regarded as egotism when I express the belief that I am the originator of the first successful weapon of the kind ever invented. [R.J. Gatling, quoted in Scientific American, Oct. 15, 1870]

Though in the light of subsequent advancements Gatling's invention of 1862 is not considered a modern machine-gun.

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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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gun-shy (adj.)
1849, originally of sporting dogs, from gun (n.) + shy (adj.).
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gun-metal (n.)
type of bronze or other alloy formerly used in the manufacture of light cannons (since superseded by steel), 1540s, from gun (n.) + metal. Used attributively of a dull blue-gray color since 1905.
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Gatling gun (n.)

1864, named for its designer, U.S. inventor Richard Jordan Gatling (1818-1903); patented by 1862 but not used in American Civil War until the Petersburg campaign of June 1864 as an independent initiative by U.S. Gen. Ben Butler.

For the first time in this war, the Gatling gun was used by Butler in repelling one of Beauregard's midnight attacks. Dispatches state that it was very destructive, and rebel prisoners were very curious to know whether it was loaded all night and fired all day. [Scientific American, June 18, 1864]
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gun-slinger (n.)
1916, American English, from gun (n.) + agent noun from sling (v.).
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