Etymology
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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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rain-cloud (n.)

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

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cloudlet (n.)

"a small cloud," 1788, from cloud (n.) + diminutive suffix -let.

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unclouded (adj.)
1590s, from un- (1) "not" + past participle of cloud (v.).
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cloudless (adj.)

"without clouds, unobscured," 1590s, from cloud (n.) + -less. Related: Cloudlessly; cloudlessness.

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becloud (v.)
1590s, "cover with clouds," from be- + cloud. Figurative sense of "to obscure" is recorded from 1610s. Related: Beclouded; beclouding.
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overcloud (v.)

"to cover or overspread with clouds," also figurative, "to cover with gloom or sorrow," 1590s, from over- + cloud (v.). Related: Overclouded; overclouding.

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