Old English regn "rain," from Proto-Germanic *regna- (cf. 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" (cf. irrigate). Rain dance is from 1867; rain date in listings for outdoor events is from 1948. To know enough to come in out of the rain (usually with a negative) is from 1590s. Rainshower is Old English renscur.
Old English regnian, usually contracted to rinan; see rain (n.), and cf. 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 as rain (blessings, tears, etc.) is recorded from 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, 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 on an exposed roof?) Probably rather an extension of cats and dogs as proverbial for "strife, enmity" (1570s).
Water that condenses from water vapor in the atmosphere and falls to Earth as separate drops from clouds. Rain forms primarily in three ways: at weather fronts, when the water vapor in the warmer mass of air cools and condenses; along mountain ranges, when a warm mass of air is forced to rise over a mountain and its water vapor cools and condenses; and by convection in hot climates, when the water vapor in suddenly rising masses of warm air cools and condenses. See also hydrologic cycle.