Etymology
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shell (n.)

"hard outer covering," Middle English shel, shelle, from Old English sciell, scill, Anglian scell "seashell; eggshell," which is related to Old English scealu "shell, husk," from Proto-Germanic *skaljo "piece cut off; shell; scale" (source also of West Frisian skyl "peel, rind," Middle Low German schelle "pod, rind, egg shell," Gothic skalja "tile"), with the shared notion of "covering that splits off," from PIE root *skel- (1) "to cut." Italian scaglia "chip" is from Germanic.

Also in late Old English as "a coating or layer." The general sense of "protective outer covering of some invertebrates" is in Middle English (by c. 1400 as "house of a snail;" by 1540s in reference to a tortoise or turtle); the meaning "outer layer of a nut" (or a fruit considered as a nut) is by mid-14c. With notion of "mere exterior," hence "empty or hollow thing" by 1650s. The meaning "hollow framework" is from 1791; that of "structure for a band or orchestra" is attested from 1938. To be out of (one's) shell "emerged into life" is by 1550s.

Military use for "explosive projectile" is by 1640s, first of hand grenades, and originally in reference to the metal case in which the gunpowder and shot were mixed; the notion is of a "hollow object" filled with explosives. Hence shell shock, "traumatic reaction to the stress of battle," recorded by 1915.

Shell game "a swindle" is from 1890, from a version of the three-card game played with a pea and walnut shells.

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shell (v.)

1560s, "to remove (a nut, etc.) from its shell," from shell (n.). The general sense of "remove or strip off the outer covering of" is by 1690s. It also can mean "enclose in a case" (1630s). The military meaning "bombard with shells" is attested by 1856. To shell out "disburse, hand over, deliver" (1801) is a figurative slang use from the image of extracting nuts. Related: Shelled; shelling.

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gas (n.1)

1650s, from Dutch gas, probably from Greek khaos "empty space" (see chaos). The sound of Dutch "g" is roughly equivalent to that of Greek "kh." First used by Flemish chemist J.B. van Helmont (1577-1644), probably influenced by Paracelsus, who used khaos in an occult sense of "proper elements of spirits" or "ultra-rarified water," which was van Helmont's definition of gas.

Hunc spiritum, incognitum hactenus, novo nomine gas voco ("This vapor, hitherto unknown, I call by a new name, 'gas.'") [Helmont, Ortus Medicinae]

Modern scientific sense began 1779, with later secondary specialization to "combustible mix of vapors" (1794, originally coal gas); "anesthetic" (1894, originally nitrous oxide); and "poison gas" (1900). Meaning "intestinal vapors" is from 1882. "The success of this artificial word is unique" [Weekley]. Slang sense of "empty talk" is from 1847; slang meaning "something exciting or excellent" first attested 1953, from earlier hepster slang gasser in the same sense (1944). Gas also meant "fun, a joke" in Anglo-Irish and was used so by Joyce (1914). Gas-works is by 1817. Gas-oven is from 1851 as a kitchen appliance; gas-stove from 1848.

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gas (v.)

1886, "to supply with (illuminating) gas," from gas (n.1). Sense of "poison with gas" is from 1889 as an accidental thing, from 1915 as a military attack. In old slang also "talk nonsense, lie to." Related: Gassed; gassing; gasses.

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gas (n.2)

short for gasoline, American English, by 1905. Gas-pump is from 1925; gas-pedal "automobile accelerator" is by 1908; gas-station "fueling station for an automobile" is from 1916.

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clam-shell (n.)

c. 1500, "the shell of a clam;" see clam (n.) + shell (n.). As "hinged iron box or bucket used in dredging" from 1877.

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hard-shell (adj.)

1838 of Baptists (figuratively); 1798 of clams; see hard (adj.) + shell (n.). Hard-shelled is from 1610s.

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gas-guzzler (n.)

car with low fuel-efficiency, 1973, American English, from gas (short for gasoline) + guzzler.

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gas-mask (n.)

1915, from (poison) gas (n.1) + mask (n.).

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gas-house (n.)

also gashouse, 1880 as a power-generating station, from gas (n.1) + house (n.). By 1926, emblematic of a run-down district of a U.S. city, a typical abode of criminals and gangsters.

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