Online Etymology Dictionary

rain (v.): 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 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).
rain (n.): Old English regn "rain," 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 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.
rain-out (n.): also rain out, rainout, 1947, from rain (v.) + out (adv.). Of baseball games, to be rained out is attested from 1928.
rain-cloud (n.): also raincloud, 1800, from rain (n.) + cloud (n.).
rain forest (n.): 1899, apparently a loan-translation of German Regenwald, coined by A.F.W. Schimper for his 1898 work "Pflanzengeographie."
raincheck (n.): also rain-check, rain check, 1884; see rain (n.) + check (n.1). Originally of tickets to rained-out baseball games.
raincoat (n.): also rain-coat, 1821, from rain (n.) + coat (n.).
rainproof (adj.): also rain-proof, 1788, from rain (n.) + proof (n.).
rainfall (n.): also rain-fall, "amount of precipitation that falls as rain," 1839, from rain (n.) + fall (n.).
rainmaker (n.): also rain-maker, 1775, in reference to American Indian tribal magicians, from rain (n.) + agent noun of make (v.).
rainstorm (n.): 1804, from rain (n.) + storm (n.).
raindrop (n.): Old English rendropa; see rain (n.) + drop (n.).
rainy (adj.): Old English renig; see rain (n.) + -y (2).
rainwater (n.): Old English renwæter; see rain (n.) + water (n.1).
rainbow (n.): Old English renboga; see rain (n.) + bow (n.). Common Germanic compound (Old Frisian reinboga, Old Norse regnbogi, Swedish regenbåge, Dutch regenboog, German Regenbogen). Rainbow trout (1876, American English) so called for its resplendent colors. Old English also had scurboga "shower-bow."
irrigate (v.): "supply land with water," 1610s, from Latin irrigatus, past participle of irrigare "lead water to, refresh, irrigate, flood," from assimilated form of in- "into, in, on, upon" (see in- (2)) + rigare "to water, to moisten," of uncertain origin. Perhaps [Watkins] from PIE *reg- (2) "moist" (see rain (n.)). De Vaan offers as possibilities the root of regere "to direct, lead," on the notion of leading water onto the fields, or to the root of rigere "be stiff," literally "stretch." The first better suits the sense, but has phonetic problems.

Related: Irrigated; irrigating. In Middle English it was an adjective, "watered, flooded" (mid-15c.). Other adjectival forms have been irriguous (1650s), irrigative (1842), irrigatorial (1867).
acid (adj.): 1620s, "of the taste of vinegar," from French acide (16c.) or directly from Latin acidus "sour, sharp," adjective of state from acere "to be sour," from PIE root *ak- "sharp, pointed" (see acrid). Figurative use from 1775; applied to intense colors from 1916. Acid test is American English, 1892, from the frontier days, when gold was distinguished from similar metals by application of nitric acid. Acid rain is first recorded 1859 in reference to England.
hyetal (adj.): "of or relating to rain," 1855, American English, from Greek hyetos "rain" (see hyeto-) + -al (1).
hyeto-: word-forming element in science meaning "rain," from Greek hyetos "rain," from hyein "to rain," from PIE root *seue- (2) "take liquid" (see sup (v.2)).
imbrication (n.): "an overlapping of edges" (as of roof tiles, etc.), 1640s, from French imbrication, noun of action from stem of Latin imbricare "to cover with tiles," from imbricem (nominative imbrex) "curved roof tile used to draw off rain," from imber (genitive imbris) "rain, heavy rain; rainwater," from PIE *ombh-ro- "rain" (source also of Sanskrit abhra "cloud, thunder-cloud, rainy weather," Greek ombros "rain, a shower"), from root *nebh- "moist; water" (see nebula).
plover (n.): c. 1300, from Anglo-French plover, Old French pluvier, earlier plovier (c. 1200), from Vulgar Latin *plovarius, literally "belonging to rain," from Latin pluvia "rain (water)" from pluere "to rain," from PIE root *pleu- "to flow" (see pluvial). Perhaps so called because the birds' migration arrival coincides with the start of the rainy season, or from its supposed restlessness when rain approaches.
embrocate (v.): 1610s, from Medieval Latin embrocatus, past participle of embrocare, from Late Latin embrocha, from Greek embrokhe "lotion, fomentation," from embrekhein "to soak in, foment," from assimilated form of en (see en- (2)) + brekhein "to water, wet, rain, send rain," related to brokhe "rain," from PIE root *mergh- "to wet, sprinkle, rain." Related: Embrocated; embrocating; embrocation (early 15c.).
pluvial (adj.): 1650s, "pertaining to rain," from French pluvial (12c.), from Latin pluvialis "pertaining to rain, rainy, rain-bringing," from (aqua) pluvia "rain (water)," from fem. of pluvius "rainy," from plovere "to rain," from PIE root *pleu- "to flow, to swim" (source also of Sanskrit plavate "navigates, swims;" Greek plynein "to wash," plein "to navigate," ploein "to float, swim," plotos "floating, navigable;" Armenian luanam "I wash;" Old English flowan "to flow;" Old Church Slavonic plovo "to flow, navigate;" Lithuanian pilu, pilti "to pour out," plauju, plauti "to swim, rinse").
aquatic (adj.): late 15c., from Middle French and Old French aquatique (13c.), from Latin aquaticus "growing in water; bringing rain," from aqua "water" (see aqua-)
lavish (adj.): "spending or bestowing profusely," mid-15c., from Middle French lavasse (n.) "torrent of rain, deluge" (15c.), from Old French lavache, from laver "to wash," from Latin lavare "to wash" (see lave). Related: Lavishly; lavishness.
nimbus (n.): 1610s, "bright cloud surrounding a god," from Latin nimbus "cloud," perhaps related to nebula "cloud, mist" (see nebula). Meaning "halo" is first recorded c. 1730. Meteorological sense of "a rain cloud" is from 1803.
dag (n.): "thin rain, drizzle, wet fog," late 17c., from a Scandinavian source such as Old Norse dögg, plural daggir "dew," from Proto-Germanic *daowo- (source of Old English deaw; see dew).
urine (n.): c. 1300, from Old French orine, urine (12c.) and directly from Latin urina "urine," from PIE *ur- (source also of Greek ouron "urine"), variant of root *we-r- "water, liquid, milk" (source also of Sanskrit var "water," Avestan var "rain," Lithuanian jures "sea," Old English wær, Old Norse ver "sea," Old Norse ur "drizzling rain"), related to *eue-dh-r (see udder).
sparse (adj.): 1727, from Latin sparsus "scattered," past participle of spargere "to scatter, spread," from PIE root *(s)preg- (2) "to jerk, scatter" (source also of Sanskrit parjanya- "rain, rain god," Avestan fra-sparega "branch, twig," literally "that which is jerked off a tree," Old Norse freknur "freckles," Swedish dialectal sprygg "brisk, active," Lithuanian sprogti "shoot, bud," Old Irish arg "a drop"). The word is found earlier in English as a verb, "to scatter abroad" (16c.). Related: Sparsely; sparseness.
acquainted (adj.): early 13c., "personally known;" past participle adjective from acquaint (v.). Of skills, situations, etc., from late 15c.
I have been one acquainted with the night.
I have walked out in rain — and back in rain.
I have outwalked the furthest city light.

[Robert Frost, from "Acquainted with the Night"]
toad (n.): c. 1300, from late Old English tadige, tadie, of unknown origin and according to OED with no known cognates outside English. Applied to loathsome persons from 1560s. Toad-strangler "heavy rain" is from 1919, U.S. Southern dialectal.
sprinkle (v.): late 14c. (implied in sprinkled), frequentative of sprenge (see spring (v.)) or via Middle Dutch, Middle Low German sprenkel "spot, speck," from PIE root *(s)preg- "to jerk, scatter" (source also of Latin spargere "to scatter, sprinkle"). The meaning "rain lightly" is first recorded 1778.
wasteland (n.): 1825 as one word, from waste (adj.) + land (n.). Figurative sense is attested from 1868. Eliot's poem is from 1922.
April is the cruellest month, breeding
Lilacs out of the dead land, mixing
Memory and desire, stirring
Dull roots with spring rain.
flood-gate (n.): early 13c. in the figurative sense "opportunity for a great venting" (especially with reference to tears or rain); literal sense is mid-15c. (gate designed to let water in or keep it out as desired, especially the lower gate of a lock); from flood (n.) + gate (n.).
imbricate (v.): "to lay one over another" (as shingles, etc.), 1704 (implied in imbricated), from Latin imbricatus "covered with tiles," past participle of imbricare "to cover with rain tiles" (see imbrication). As an adjective from 1650s. Related: Imbricated; imbricating.
Hyades: star cluster in constellation Taurus (generally pictured as forming the head of the bull), late 14c., from Greek Hyades, popularly explained by the ancients as "rain-bringers" (from hyein "to rain"), because wet weather supposedly began coincidentally with their heliacal rising; but probably rather from hys "swine" (the popular Latin word for the star-group was Suculae "piglets, little pigs"), from PIE *su- "pig" (see sow (n.)). Grimm ("Teutonic Mythology") lists the Anglo-Saxon glosses of Hyades as Raedgastran, Raedgasnan, Redgaesrum.
umbrella (n.): "hand-held portable canopy which opens and folds," c. 1600, first attested in Donne's letters, from Italian ombrello, from Late Latin umbrella, altered (by influence of umbra) from Latin umbella "sunshade, parasol," diminutive of umbra "shade, shadow" (see umbrage).

A sunshade in the Mediterranean, a shelter from the rain in England; in late 17c. usage, usually as an Oriental or African symbol of dignity. Said to have been used by women in England from c. 1700; the use of rain-umbrellas carried by men there traditionally is dated to c. 1750, first by Jonas Hathaway, noted traveler and philanthropist. Figurative sense of "authority, unifying quality" (usually in a phrase such as under the umbrella of) is recorded from 1948.
sup (v.2): "to sip, to take into the mouth with the lips," Old English supan (West Saxon), suppan, supian (Northumbrian) "to sip, taste, drink, swallow" (strong verb, past tense seap, past participle sopen), from Proto-Germanic *supanan (source also of Old Norse supa "to sip, drink," Middle Low German supen, Dutch zuipen "to drink, tipple," Old High German sufan, German saufen "to drink, booze"), from PIE *sub-, possibly an extended form of root *seue- (2) "to take liquid" (source also of Sanskrit sunoti "presses out juice," soma; Avestan haoma, Persian hom "juice;" Greek huetos "rain," huein "to rain;" Latin sugere "to suck," succus "juice, sap;" Lithuanian sula "flowing sap;" Old Church Slavonic soku "sap," susati "suck;" Middle Irish suth "sap;" Old English seaw "sap").
hail (n.): "frozen rain, pellets of ice falling in showers," Old English hægl, hagol (Mercian hegel) "hail, hailstorm," also the name of the rune for H, from Proto-Germanic *haglaz (source also of Old Frisian heil, Old Saxon, Old High German hagal, Old Norse hagl, German Hagel "hail"), probably from PIE *kaghlo- "pebble" (source also of Greek kakhlex "round pebble").
tampion (n.): early 15c., "plug, bung," from Middle French tampon (15c.), nasalized variant of Old French tapon "piece of cloth to stop a hole" (14c.), a suffixed form of Frankish *tappo "stopper, plug," related to Old High German zapfo and Old English tæppa "stopper" (see tap (n.1)). Meaning "wooden plug for the muzzle of a gun" (to keep out rain or seawater) is recorded from 1620s.
freshet (n.): 1590s, "stream of fresh water; stream flowing into the sea," from obsolete fresh (n.) "a stream in flood" (1530s), also "mingling of fresh and salt water," from fresh (adj.1). Old English had fersceta in the same sense. Meaning "small flood or increased flow of an ebb tide caused by rain or melting snow" is from 1650s.
aqua-: word-forming element meaning "water," from Latin aqua "water; the sea; rain," cognate with Proto-Germanic *akhwo, source of Old English ea "river," Gothic ahua "river, waters," Old Norse Ægir, name of the sea-god, Old English ieg "island;" all from PIE *akwa- "water" (source also of Sanskrit ap "water," Hittite akwanzi "they drink," Lithuanian uppe "a river").
monsoon (n.): 1580s, "trade wind of the Indian Ocean," from Dutch monssoen, from Portuguese monçao, from Arabic mawsim "time of year, appropriate season" (for a voyage, pilgrimage, etc.), from wasama "he marked." When it blows from the southwest (April through October) it brings heavy rain, hence "heavy episode of rainfall during the rainy season" (1747).
precipitation (n.): late 15c., "a casting down" (of the evil angels from heaven), also, in alchemy "separation of a solid substance from a solution," from Middle French precipitation (15c.) and directly from Latin praecipitationem (nominative praecipitatio) "act or fact of falling headlong, haste," noun of action from past participle stem of praecipitare "fall, be hasty," from praeceps "steep" (see precipice). Meaning "sudden haste" is c. 1500. Meaning "act of falling from a height" is attested from 1610s. Meteorological sense of "rain, snow, dew, etc." is from 1670s.
siccative (adj.): 1540s, from Late Latin siccativus "drying, siccative," from Latin siccatus, past participle of siccare "to dry, make dry; dry up," from siccus "dry, thirsty; without rain," from PIE root *seikw- "to flow out" (source also of Avestan hiku- "dry," Greek iskhnos "dry, withered," Lithuanian seklus "shallow," Middle Irish sesc "dry," Sanskrit sincati "makes dry"). The noun is first recorded 1825.
haze (v.): "subject (someone) to cruel horseplay," 1850, American English student slang, from earlier nautical sense of "harass with work, punish by keeping at unpleasant and unnecessary hard labor" (1840), perhaps from hawze "terrify, frighten, confound" (1670s), from Middle French haser "irritate, annoy" (mid-15c.), which is of unknown origin. Related: Hazed; hazing.
All hands were called to "come up and see it rain," and kept on deck hour after hour in a drenching rain, standing round the deck so far apart as to prevent our talking with one another, with our tarpaulins and oil-cloth jackets on, picking old rope to pieces or laying up gaskets and robands. This was often done, too, when we were lying in port with two anchors down, and no necessity for more than one man on deck as a look-out. This is what is called "hazing" a crew, and "working their old iron up." [Dana, "Two Years before the Mast," 1842]
gargoyle (n.): "grotesque carved waterspout," connected to the gutter of a building to throw down water clear of the wall, common in 13c.-16c. buildings; late 13c., gargoile, also garguile, gargule, etc., "carved mouth of a rain spout, a gargoyle," from Old French gargole, gargoule "throat;" also "carved downspout," in the form of a serpent or some other fanciful shape, also from Medieval Latin gargola, gargulio (see gargle (v.)). "An archaic spelling, retained in books; better gargoil or, in more modern form gargel" [Century Dictionary].
immense (adj.): "great beyond measure," early 15c., from Old French immense (mid-14c.), from Latin immensus "immeasurable, boundless," also used figuratively, from assimilated form of in- "not, opposite of" (see in- (1)) + mensus "measured," past participle of metiri "to measure" (see measure (v.)). A vogue word in 18c., and mocked as such:
For instance, a long while every thing was immense great and immense little, immense handsome and immense ugly. Miss Tippet from the cloisters, could not drink tea with Master Parchment at the White Conduit-house, unless it was an immense fine day, yet probably it might rain so immense, there was no going without a coach. ["Town and Country Magazine" (in "Annual Register" for 1772)]
mask (n.): 1530s, from Middle French masque "covering to hide or guard the face" (16c.), from Italian maschera, from Medieval Latin masca "mask, specter, nightmare," of uncertain origin, perhaps from Arabic maskharah "buffoon, mockery," from sakhira "be mocked, ridiculed." Or via Provençal mascarar, Catalan mascarar, Old French mascurer "to black (the face)," perhaps from a Germanic source akin to English mesh (q.v.). But compare Occitan mascara "to blacken, darken," derived from mask- "black," which is held to be from a pre-Indo-European language, and Old Occitan masco "witch," surviving in dialects; in Beziers it means "dark cloud before the rain comes." [See Walther von Wartburg, "Französisches Etymologisches Wörterbuch: Eine Darstellung galloromanischen sprachschatzes"]. Figurative use by 1570s.
louver (n.): also louvre, early 14c., "domed turret-like structure atop a building to disperse smoke and admit light," from Old French lovier, of uncertain origin. One theory connects it to Medieval Latin *lodarium, which might be from a Germanic source (compare Old High German louba "upper room, roof;" see lobby). Another suggests it is from French l'ouvert, literally "the open place," from le, definite article, + past participle of ouvrir "to open." Meaning "overlapping strips in a window (to let in air but keep out rain)" first recorded 1550s. The form has been influenced by apparently unrelated French Louvre, the name of the palace in Paris, which is said to be so named because its builder, Philip Augustus, intended it as a wolf kennel. Related: Louvered.