Etymology
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sky (n.)

c. 1200, "a cloud," from Old Norse sky "cloud," from Proto-Germanic *skeujam "cloud, cloud cover" (source also of Old English sceo, 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."

Meaning "upper regions of the air" is attested from c. 1300; 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, originally "the clouds." Sky-high is from 1812; phrase the sky's the limit is attested from 1908. Sky-dive first recorded 1965; sky-writing is from 1922.

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sky (v.)

"to raise or throw toward the skies," 1802, from sky (n.).

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skyline (n.)

"horizon," 1824, from sky (n.) + line (n.).

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skylark (n.)

the common European lark, 1680s, from sky (n.) + lark (n.1). So called because it sings as it mounts toward the sky in flight.

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skyclad (adj.)

also sky-clad, "naked," 1909, from sky (n.) + clad. Perhaps translating Sanskrit digam-bara "having the four quarters for clothing."

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skylight (n.)

1670s, "light from the sky," from sky (n.) + light (n.). Meaning "small opening in a roof to admit light" is recorded from 1680s. Related: Sky-lit.

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skyjack (v.)

"to hijack an airplane," 1961, apparently coined in New York "Mirror" headlines, from sky (n.) + second element of hijack (q.v.).

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Skylab (n.)

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

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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 actual device for lifting things into the air.

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skyrocket (n.)

1680s, type of firework, from sky (n.) + rocket (n.2). The verb, in the figurative sense of "to rise abruptly and rapidly" (often with suggestion of 'and then explode and vanish'") is attested from 1895.

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