Online Etymology Dictionary

rapprochement (n.): 1809, from French rapprochement "reunion, reconciliation," from rapprocher "bring near," from Old French re- "back, again" (see re-) + aprochier (see approach).
rapscallion (n.): 1690s, alteration of rascallion (1640s), a fanciful elaboration of rascal (q.v.). It is the parallel term of now-extinct rampallion (1590s), from Middle English ramp (n.) "ill-behaved woman" (mid-15c.), which is probably connected to the definition of romp in Johnson's Dictionary (1755) as "a rude, awkward, boisterous, untaught girl."
rapt (adj.): c.1400, "carried away" (in an ecstatic trance), from Latin raptus, past participle of rapere "seize, carry off" (see rape (v.)). Sense of "engrossed" first recorded c.1500. As a past participle adjective, in English it spawned the back-formed verb rap "to affect with rapture," which was common c.1600-1750. The figurative sense is from the notion of "carried up into Heaven (bodily or in a dream)," as in a saint's vision.
raptor (n.): c.1600, "raviser, abductor," from Latin raptor, agent noun from past participle stem of rapere (see rapid). Ornithological use is from 1873 (1823 in Latin plural Raptores).
raptorial (adj.): "predatory," 1825, from Latin raptor "robber," from rapt-, past participle stem of rapere "seize" (see rapid).
rapture (n.): c.1600, "act of carrying off," from Middle French rapture, from Medieval Latin raptura "seizure, rape, kidnapping," from Latin raptus "a carrying off" (see rapt). Originally of women and cognate with rape (v.). Sense of "spiritual ecstasy" first recorded 1620s.
rapturous (adj.): 1670s, from rapture + -ous. Related: Rapturously (1660s).
rara avis (n.): c.1600, "peculiar person," from Latin (Juvenal, Persius), literally "strange bird," from rara, fem. of rarus "rare" (see rare (adj.1)) + avis "bird" (see aviary). Latin plural is raræ aves.
rare (adj.1): "unusual," early 15c., originally "few in number and widely separated," from Old French rere "sparse" (14c.), from Latin rarus "thinly sown, having a loose texture," from PIE *er-, *ere- "to loose, split, separate" (cf. Sanskrit rte "besides, except," viralah "distant, tight, rare;" Old Church Slavonic oriti "to dissolve, destroy;" Lithuanian irti "to dissolve;" Old Church Slavonic rediku "rare;" Greek eremos "solitary"). "Few in number," hence, "unusual" (1540s). In chemistry, rare earth is from 1818.
rare (adj.2): "undercooked," 1650s, variant of Middle English rere, from Old English hrer "lightly cooked," probably related to hreran "to stir, move." Originally of eggs, not recorded in reference to meat until 1784, and according to OED, in this sense "formerly often regarded as an Americanism, although it was current in many English dialects ...."
rare (v.): "rise up," 1833, dialectal variant of rear (v.). Sense of "eager" (in raring to go) first recorded 1909. Related: Rared; raring.
rarebit: 1785, perversion of (Welsh) rabbit, as if from rare + bit. See Welsh.
raree show: "peep show contained in a box," 1680s, so called "in imitation of the foreign way of pronouncing rare show" [Johnson]. "Johnson's statement is prob. correct; the early exhibitors of peep-shows appear to have been usually Savoyards, from whom the form was no doubt adopted" [OED]. Early "peep shows" were more innocent than what usually is meant now by that word.
rarefaction (n.): c.1600, noun of action from Latin rarefacere (see rarefy).
rarefy (v.): late 14c., from Old French rarefier (14c.), from Medieval Latin rarificare, from Latin rarefacere "make rare," from rarus "rare, thin" (see rare (adj.1)) + facere "to make" (see factitious). Related: Rarefied.
rarely (adv.): from rare (adj.1) + -ly (2).
rarity (n.): 1550s, "fewness," from Middle French rarité (16c.) or directly from Latin raritas "thinness, fewness," from rarus (see rare (adj.1)). Meaning "thinness" is from 1640s; noun sense of "a rare thing or event: is from 1590s.
ras: Ethiopian title, from Amharic ras "chief, head," from Arabic ra's.
rascal (n.): early 14c., rascaile "people of the lowest class, rabble of an army," from Old French rascaille "outcast, rabble" (12c.), perhaps from rasque "mud, filth, scab, dregs," from Vulgar Latin *rasicare "to scrape" (see rash (n.)). The singular form is first attested mid-15c.; extended sense of "low, dishonest person" is from 1580s.
rash (adj.): c.1300, "nimble, quick, vigorous," a Scottish and northern word, perhaps from Old English -ræsc (cf. ligræsc "flash of lightning"), from Proto-Germanic *raskuz (cf. Middle Low German rasch, Middle Dutch rasc "quick, swift," German rasch "quick, fast"). Related to Old English horsc "quick-witted." Sense of "reckless, impetuous, heedless of consequences" is attested from c.1500.
rash (n.): "red spots on skin," 1709, perhaps from French rache "a sore," from Old French rache "ringworm," from Vulgar Latin *rasicare "to scrape" (cf. Old Provençal rascar, Spanish rascar "to scrape, scratch," Italian raschina "itch"), from Latin rasus "scraped," past participle of radere "to scrape" (see raze). The connecting notion is of itching. Sense of "any sudden outbreak or proliferation" first recorded 1820.
rasher (n.): "thin slice of bacon or ham," 1590s, of unknown origin. Perhaps from Middle English rash "to cut," variant of rase "to rub, scrape out, erase," from Old French raser (see raze). However, early lexicographer John Minsheu explained it in 1627 as a piece "rashly or hastily roasted."
Raskolnik: "dissenter from the Russian Church, an Old Believer," 1723, from Russian Raskolnik "separatise," from raskol "schism, separation." The schism was a result of reforms by Patriarch Nikon in 1667.
rasp (v.): c.1300, "to scrape," from Middle Dutch raspen, Old French rasper "to grate, rasp," from a West Germanic source (cf. Old English gehrespan) akin to the root of raffle (q.v.). Vocalic sense is from 1843. Related: Rasped; rasping.
rasp (n.): "coarse file," 1540s, from Middle French raspe, from Old French rasper "to rasp" (see rasp (v.)).
raspberry (n.): 1620s, earlier raspis berry (1540s), possibly from raspise "a sweet rose-colored wine" (mid-15c.), from Anglo-Latin vinum raspeys, origin uncertain, as is the connection between this and Old French raspe, Medieval Latin raspecia, raspeium, also meaning "raspberry." One suggestion is via Old Walloon raspoie "thicket," of Germanic origin. Meaning "rude sound" (1890) is shortening of raspberry tart, rhyming slang for fart.
Rasputin: acquired name (Russian, literally "debauchee") of Grigory Yefimovich Novykh (c.1872-1916), mystic and faith healer who held sway over court of Nicholas II of Russia. His nickname is from his doctrine of "rebirth through sin," that true holy communion must be preceded by immersion in sin. His name used figuratively in English from 1937 for anyone felt to have an insidious and corrupting influence.
raspy (adj.): 1670s, of plants; by 1821 of voices, from rasp + -y (2).
rasta: 1955; see Rastafarian.
Rastafarian: 1953 (Rastafarite), from Rastafari, Jamaican religion built around writings of Marcus Garvey and belief that Haile Selassie (1892-1975), former emperor of Ethiopia, was God. From Ras Tafari, Selassie's title from 1916 to his accession in 1936, from Amharic ras "chief, head" (from Arabic ra's) + tafari "to be feared." Shortened form Rasta is recorded from 1955.
Rastafarianism: 1964, from Rastafarian + -ism. Rastafarism is attested from 1955.
rastaquouere (n.): 1883, from French rastaquouère "social intruder, upstart" (especially one of exaggerated manners and dress, from a Mediterranean or South American country), thus "dashing but untrustworthy foreigner," from South American Spanish rastacuero "upstart." Short form rasta attested from 1905.
raster (n.): 1934 in electrical engineering, from German Raster "screen, frame," from Latin rastrum "rake," from rasum, from rodere "to scrape" (see raze). Related: Rasterization; rasterize.
rat (n.): Old English ræt, of uncertain origin. Similar words are found in Celtic (Gaelic radan), Romanic (Italian ratto, Spanish rata, French rat) and Germanic (Middle Low German rotte, German ratte) languages, but connection is uncertain and origin unknown. Perhaps from Vulgar Latin *rattus, but Weekley thinks this is of Germanic origin, "the animal having come from the East with the race-migrations" and the word passing thence to the Romanic languages. American Heritage and Tucker connect Old English ræt to Latin rodere and thus PIE *red- "to scrape, scratch, gnaw," source of rodent (q.v.).

Klein says there is no such connection and suggests a possible cognate in Greek rhine "file, rasp." Weekley connects them with a question mark and Barnhart writes, "the relationship to each other of the Germanic, Romance, and Celtic words for rat is uncertain." OED says "probable" the rat word spread from Germanic to Romanic, but takes no position on ultimate origin.

Middle English common form was ratton, from augmented Old French form raton. Sense of "one who abandons his associates" (1620s) is from belief that rats leave a ship about to sink or a house about to fall and led to meaning "traitor, informant" (1902; verb 1910). Interjection rats is American English, 1886. To smell a rat is c.1550. _____-rat, "person who frequents _____" (in earliest reference dock-rat) is from 1864. Rat-pack "juvenile gang" is from 1951.
rat fink (n.): also ratfink, 1963, teen slang, see rat (n.) + fink (n.).
rat race (n.): also rat-race, "competitive struggle," 1934, from rat (n.) + race (1.).
rat-a-tat: 1680s, echoic.
ratafia (n.): liqueur flavored with kernels of cherries, apricots, etc., 1690s, from French (17c.), of unknown origin.
ratatouille (n.): 1877, from French, first element uncertain, second element evidently touiller "to stir up."
ratchet (v.): 1852, from ratchet (n.). Transferred sense attested by 1977. Related: Ratcheted; ratcheting.
ratchet (n.): 1650s, from French rochet "bobbin, spindle," from Italian rocchetto "spool, ratchet," diminutive of rocca "distaff," possibly from a Germanic source (cf. Old High German rocko "distaff," Old Norse rokkr), from Proto-Germanic *rukka-, from PIE root *rug- "to spin." Cf. rocket (2). Spelling in English influenced by German Rätsche "ratchet."
rate (n.): "estimated value or worth," early 15c., from Middle French rate "price, value," from Medieval Latin rata (pars) "fixed (amount)," from Latin rata "fixed, settled," fem. past participle of reri "to reckon, think" (see reason (n.)). Meaning "degree of speed" (prop. ratio between distance and time) is attested from 1650s. Currency exchange sense first recorded 1727. First-rate, second-rate, etc. are 1640s, from British Navy division of ships into six classes based on size and strength. Phrase at any rate originally (1610s) meant "at any cost;" weakened sense of "at least" is attested by 1760.
rate (v.1): "to scold," late 14c., probably from Old French reter "to impute blame," from Latin reputare "to count over, reflect," in Vulgar Latin, "to impute, blame" (see reputation). Related: Rated; rating.
rate (v.2): "estimate the worth or value of," 1590s, from rate (n.). Related: Rated; rating. .
rather (adv.): Old English hraþor "more quickly, earlier, sooner," also "more readily," comparative of hraþe, hræþe "quickly," related to hræð "quick," from Proto-Germanic *khrathuz (cf. Old Norse hraðr, Old High German hrad). The base form rathe was obsolete by 18c. except in poetry; superlative rathest fell from use by 17c. Meaning "more willingly" is recorded from c.1300; sense of "more truly" is attested from late 14c.
rathole: also rat-hole, 1812 in figurative sense of "nasty, messy place;" rat + hole (n.).
rathskeller (n.): 1900, from German ratskeller, earlier rathskeller, "a cellar in a German town hall in which beer is sold," from rat "council" (see read (v.)) + keller "cellar."
ratification (n.): mid-15c., from Old French ratification (14c.) or directly from Medieval Latin ratificationem (nominative ratificatio, 13c.), noun of action from past participle stem of Latin ratificare (see ratify).
ratify (v.): mid-14c., from Old French ratifier (late 13c.), from Medieval Latin ratificare "confirm, approve," literally "fix by reckoning," from Latin ratus "fixed, valid" (past participle of reri "to reckon, think") + root of facere "to make" (see factitious). Related: Ratified; ratifying.
rating (n.): action of verb "to rate" 1530s, from rate (n.). Ratings of TV programs, originally radio programs, began 1930 in U.S. under system set up by U.S. pollster and market researcher Archibald M. Crossley (1896-1985), and were called Crossley ratings or Crossleys until ratings began to be preferred c.1947.