Etymology
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sky (n.)

mid-13c. (c. 1200 as a surname), skie, sci, skei, "a cloud," from Old Norse sky "cloud," from Proto-Germanic *skeujam "cloud, cloud cover" (source also of Old English sceo (Middle English sceu) "the sky, the heavens," Old Saxon scio "cloud, region of the clouds, sky;" Old High German scuwo, Old English scua, Old Norse skuggi "shadow;" Gothic skuggwa "mirror"), from PIE root *(s)keu- "to cover, conceal."

The meaning "upper regions of the air; region of clouds, wind, and rain; the heavens, the firmament" is attested from c. 1300; it replaced native heofon in this sense (see heaven). In Middle English, the word can still mean both "cloud" and "heaven," as still in the skies (c. 1300), originally "the clouds."

Sky-high "as high as the sky" is from 1812; optimistic phrase the sky's the limit is attested from 1908. Sky-writing is from 1922. Sky-diving "sport or activity of jumping from an aircraft and free-falling before landing by parachute" is attested from 1959 (sky-diver by 1961; sky-dive (v.) by 1965).

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sky (v.)

"to raise or throw toward the skies," 1802, from sky (n.). By 1865 in reference to paintings hung near the ceiling in an exhibit. Related: Skyed; skyer; skying.

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sky-pilot (n.)

by 1877 as a slang term for a clergyman, from sky (n.) + pilot (n.). Often, but not originally, a sailor's term for a navy chaplain or any who has spiritual charge of seamen. Also "a balloonist" (by 1854).

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sky-blue (adj.)

"of a luminous blue suggesting sky color" (but often very unlike it), by 1738, from sky (n.) + blue (n.). Sky-colored is attested from 1580s; simple sky for "sky-blue" is from 1660s. Compare cerulean.

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sky-clad (adj.)

also skyclad, "naked, clothed in space," by 1909 (Webster), from sky (n.) + clad. OED says "esp. in Witchcraft," but perhaps translating Sanskrit digam-bara "having the four quarters for clothing;" sky-clothed translating the Sanskrit term is attested by 1924.

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skylark (n.)

alsosky-lark, a popular name of the common European lark, 1680s, from sky (n.) + lark (n.1). So called because it sings as it mounts skyward in flight.

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skyey (adj.)

"like the sky," c. 1600, from sky (n.) + -y (2). First attested in Shakespeare, who elsewhere used skyish. Sky-like is attested from 1590s.

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skyhook (n.)

also sky-hook, "imaginary device to hold things up," 1915, originally aviators' jargon, from sky (n.) + hook (n.). Applied from 1935 to helicopters, balloons, etc. used as actual device for lifting things off the surface.

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Skylab (n.)

name of a U.S. space program, the name first attested 1970; the thing launched 1973, abandoned 1974, fell to earth 1979. From sky (n.) + lab (n.).

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skyline (n.)

also sky-line, "horizon; place where the sky and the earth, or objects on it, seem to meet," 1824, from sky (n.) + line (n.). Originally often in reference to paintings.

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