Etymology
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rain (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.

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rain (v.)

"fall in drops through the air," Middle English reinen, from Old English regnian, usually contracted to rinan; see rain (n.), and compare Old Norse rigna, Swedish regna, Danish regne, Old High German reganon, German regnen, Gothic rignjan. Related: Rained; raining. Transferred and figurative use, of other things that fall in showers or drops (blessings, tears, etc.), is by c. 1200.

To rain on (someone's) parade is attested from 1941. Phrase to rain cats and dogs is attested from 1738 (variation rain dogs and polecats is from 1650s), of unknown origin and signification, despite intense speculation. One of the less likely suggestions is pets sliding off sod roofs when the sod got too wet during a rainstorm. (Ever see a dog react to a rainstorm by climbing up an exposed roof?) Probably rather an extension of cats and dogs as proverbial for "strife, enmity" (1570s).

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rain-cloud (n.)

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

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rain-proof (adj.)

also rainproof, "not admitting the entrance of rain or penetration by it," 1788, from rain (n.) + proof (n.).

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rain-out (n.)

also rain out, rainout, "cancellation or interruption of an outdoor event due to rain," 1947, from the verbal phrase; see rain (v.) + out (adv.). Of baseball games, to be rained out "cancelled because of rain" is attested from 1928.

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rainstorm (n.)

"a storm of rain," 1804, from rain (n.) + storm (n.).

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raindrop (n.)

"a drop of rain," Middle English rein-drope, from Old English rendropa; see rain (n.) + drop (n.).

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raincoat (n.)

also rain-coat, "coat worn as protection against rain," 1821, from rain (n.) + coat (n.).

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rainfall (n.)

also rain-fall, by 1850 as "amount of precipitation that falls as rain, from rain (n.) + fall (n.). By 1858 as "a falling of rain."

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rainwater (n.)

"water that has fallen from the clouds as rain and has not sunk into the earth," Old English renwæter; see rain (n.) + water (n.1).

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