Etymology
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cirrus (n.)

1708, "curl-like fringe or tuft," from Latin cirrus "a lock of hair, tendril, curl, ringlet of hair; the fringe of a garment." In meteorology, in reference to light, fleecy clouds, attested from 1803; so called from fancied resemblance of shape.

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cloud (n.)

Old English clud "mass of rock, hill," related to clod.

The modern sense "rain-cloud, mass of evaporated water visible and suspended in the sky" is a metaphoric extension that begins to appear c. 1300 in southern texts, based on similarity of cumulus clouds and rock masses. The usual Old English word for "cloud" was weolcan (see welkin). In Middle English, skie also originally meant "cloud." The last entry for cloud in the original rock mass sense in Middle English Compendium is from c. 1475.

The four fundamental types of cloud classification (cirrus, cumulus, stratus, nimbus) were proposed by British amateur meteorologist Luke Howard (1772-1864) in 1802.

Meaning "cloud-like mass of smoke or dust" is from late 14c. Figuratively, as something that obscures, darkens, threatens, or casts a shadow, from c. 1300; hence under a cloud (c. 1500). In the clouds "removed from earthly things; obscure, fanciful, unreal" is from 1640s. Cloud-compeller translates (poetically) Greek nephelegereta, a Homeric epithet of Zeus.

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cloud (v.)

early 15c., "overspread with clouds, cover, darken," from cloud (n.). From 1510s as "to render dim or obscure;" 1590s as "to overspread with gloom." Intransitive sense of "become cloudy" is from 1560s. Related: Clouded; clouding.

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cloud-burst (n.)

also cloudburst, "violent downpour of much rain over a small area," 1817, American English, from cloud (n.) + burst (n.). It parallels German Wolkenbruch.

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cloud nine (n.)

by 1950, sometimes also cloud seven (1956, perhaps by confusion with seventh heaven), American English, of uncertain origin or significance. Some connect the phrase with the 1895 International Cloud-Atlas (Hildebrandsson, Riggenbach and Teisserenc de Bort), long the basic source for cloud shapes, in which, of the ten cloud types, cloud No. 9, cumulonimbus, was the biggest, puffiest, most comfortable-looking. Shipley suggests the sense in this and other expressions might be because, "As the largest one-figure integer, nine is sometimes used for emphasis." The phrase might appear in the 1935 aviation-based play "Ceiling Zero" by Frank Wilbur Wead.

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rain-cloud (n.)

also raincloud, "cloud from which rain falls," 1800, from rain (n.) + cloud (n.).

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Oort cloud (n.)

in reference to a hypothetical cloud of small objects beyond Pluto that become comets, proposed 1949 by Dutch astronomer Jan Hendrick Oort (1900-1992), and named for him by 1968.

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Cloud Cuckoo Land 

imaginary city built in air, 1830, translating Aristophanes' Nephelokokkygia in "The Birds" (414 B.C.E.). Cloud-land "place above the earth or away from the practical things of life, dreamland, the realm of fancy" is attested from 1840.

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cirrocumulus (n.)

also cirro-cumulus, type of cloud having the characteristics of both cirrus and cumulus clouds, 1803, from combining form of cirrus + cumulus.

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cirro- 

word-forming element meaning, in meteorology, "involving cirrus clouds," and, in biology, "involving a tendril or tendrils," from combining form of Latin cirrus "lock of hair, tendril" (see cirrus).

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