cloudyoudaoicibaDictYouDict[cloud 词源字典]
cloud: [OE] In Old English the word for ‘cloud’ was weolcen (whence modern English welkin, a poetical term for ‘sky’), which is related to German wolke ‘cloud’. At that time Old English clūd, the ancestor of cloud, meant ‘mass of rock, hill’ (it is probably related to clod). As applied to ‘clouds’, presumably from a supposed resemblance between cumulus clouds and lumps of earth or rock, it dates from the 13th century.
=> clod[cloud etymology, cloud origin, 英语词源]
cloud (n.)youdaoicibaDictYouDict
Old English clud "mass of rock, hill," related to clod. Metaphoric extension to "raincloud, mass of evaporated water in the sky" is attested by c. 1200 based on similarity of cumulus clouds and rock masses. The usual Old English word for "cloud" was weolcan. In Middle English, skie also originally meant "cloud."

The four fundamental types of cloud classification (cirrus, cumulus, stratus, nimbus) were proposed by British amateur meteorologist Luke Howard (1772-1864) in 1802. Figuratively, as something that casts a shadow, from early 15c.; 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.
cloud (v.)youdaoicibaDictYouDict
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.