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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obnubilate (v.)

"to darken, cloud, overcloud," 1580s, from Latin obnibulatus, past participle of obnubilare "to cover with clouds or fog," from ob "in front of, against" (see ob-) + verb from Latin nubes "cloud," from PIE *sneudh- "fog" (see nuance). Related: Obnubilated; obnubilating. Middle English had obnubilous "obscure, indistinct" (early 15c.).

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ombro- 

word-forming element meaning "rain, rainfall; excessive moisture," from Greek ombros "shower of rain," from PIE *ombh-ro- "rain" (source also of Sanskrit abhra "cloud, thunder-cloud, rainy weather," Latin imber "rain, heavy rain; rainwater"), from root *nebh- "moist; water" (see nebula). 

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becloud (v.)

1590s, "cover with clouds," from be- + cloud. The figurative sense of "to obscure" is recorded from 1610s. Related: Beclouded; beclouding.

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thunderhead (n.)

"high-piled cumulus cloud," one likely to develop into a thunderstorm, 1861, from thunder (n.) + head (n.).

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protoplanet (n.)

"large, diffuse cloud of matter in the orbit of a young star, regarded as the preliminary state of a planet," 1949, from proto- + planet.

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nimbus (n.)

1610s, "bright cloud surrounding a divine or sacred personage," from Latin nimbus "cloud," which is perhaps related to nebula "cloud, mist" (from PIE root *nebh- "cloud"). In art, the meaning "halo around the head of a representation of a divine or sacred person" is by 1727. Figurative use is by 1860. Compare aureole.

The nimbus of God the Father is represented as of triangular form, with rays diverging from it on all sides, or in the form of two superposed triangles, or in the same form (inscribed with the cross) as that of Christ. The nimbus of Christ contains a cross more or less enriched; that of the Virgin Mary is a plain circle, or occasionally a circlet of small stars, and that of angels and saints is often a circle of small rays. When the nimbus is depicted of a square form, it is supposed to indicate that the person was alive at the time of delineation. [Century Dictionary]
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puff-ball (n.)

type of fungus, 1640s, from puff + ball (n.1). So called for discharging a cloud of spores when disturbed.

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welkin (n.)

"sky" (poetic), Old English wolcen "cloud," also "sky, heavens," from Proto-Germanic *wulk- (source also of Old Saxon wolkan, Old Frisian wolken, Middle Dutch wolke, Dutch wolk, Old High German wolka, German Wolke "cloud"), perhaps from PIE *welg- "wet" (source also of Lithuanian vilgyti "to moisten," Old Church Slavonic vlaga "moisture," Czech vlhky "damp"); but Boutkan rejects this and finds no good IE etymology.

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nebula (n.)

mid-15c., nebule "a cloud, mist," from Latin nebula, plural nebulae, "mist, vapor, fog, smoke, exhalation," figuratively "darkness, obscurity," from PIE root *nebh- "cloud."

Re-borrowed from Latin 1660s in sense of "cataracts in the eye;" astronomical meaning "luminous cloud-like patch in the heavens" is from c. 1730. As early as Herschel (1802) astronomers realized that some nebulae were star clusters, but the certain distinction of relatively nearby cosmic gas clouds from distant galaxies (as these are now properly called) was not made until the 1920s, when the latter were resolved into individual stars (and nebulae) using the new 100-inch Mt. Wilson telescope.

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