rain-cloud n.

Middle English rein, from Old English regn "rain, descent of water in drops through the atmosphere," from Proto-Germanic *regna- (source also of Old Saxon regan, Old Frisian rein, Middle Dutch reghen, Dutch regen, German regen, Old Norse regn, Gothic rign "rain"), with no certain cognates outside Germanic, unless it is from a presumed PIE *reg- "moist, wet," which may be the source of Latin rigare "to wet, moisten" (see irrigate).

Rain dance "dance performed by a tribal group in hope of summoning rain" is from 1867; rain date in listings for outdoor events, giving an alternative date should rain interrupt them on the intended day, is from 1948. To know enough to come in out of the rain (usually with a negative) "take ordinary measures for one's protection" is from 1590s. Rain-shower is Old English renscur. Rain-gauge "instrument for collecting and measuring the amount of rainfall at a given place" is by 1769.

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.