O.E.
regn, from P.Gmc.
*regna- (cf. O.S.
regan, O.N.
regn, O.Fris.
rein, M.Du.
reghen, Ger.
regen, Goth.
rign "rain"), with no certain cognates outside Gmc., unless it is from a presumed PIE
*reg- "moist, wet," which may be the source of L.
rigare "to wet, moisten" (cf.
irrigate). The verb is O.E.
regnian, usually contracted to
rinan. Transf. and fig. use of other things that fall as rain (blessings, tears, etc.) is recorded from c.1200.
Rainbow is O.E.
renboga (common Gmc. compound, cf. O.N.
regnbogi, Du.
regenboog, see
bow (n.)).
Raindrop is O.E.
rendropa; first record of
raincheck is from 1884, originally of tickets to rained-out baseball games.
Raincoat attested from 1830.
Rainmaker first recorded 1775, in ref. to tribal magicians. Phrase to
rain cats and dogs is attested from 1738 (variation
rain dogs and polecats is from 1652), of unknown origin, despite intense speculation. One of the more idiotic assertions is that it refers to pets sliding off sod roofs when the sod got too wet during a rainstorm. Ever see a cat react to a rainstorm by climbing up on an exposed roof? To
rain on (someone's) parade is attested from 1941.
[reyn] 
