Online Etymology Dictionary

rat (v.): 1812, "to desert one's party; 1864 as "to catch rats;" 1910 as "to peach on, inform on, behave dishonestly toward;" from rat (n.). Related: Ratted; ratting.
rat fink (n.): also ratfink, 1963, teen slang, see rat (n.) + fink (n.). Popularized by, and perhaps coined by, U.S. custom car builder Ed "Big Daddy" Roth (1932-2001), who made a hot-rod comic character of it, supposedly to lampoon Mickey Mouse.
rat-a-tat: 1680s, echoic, originally of a cooper hammering tubs.
rat-catcher (n.): 1590s, from rat (n.) + catcher.
rat-hole (n.): also rathole, 1812 in figurative sense of "nasty, messy place;" rat (n.) + hole (n.). As "bottomless hole" (especially one where money goes) from 1961.
rat-pack (n.): "juvenile gang," 1951, from rat (n.) + pack (n.). In reference to a Hollywood circle around Frank Sinatra, from 1958.
rat-poison (n.): "arsenic," 1799, from rat (n.) + poison (n.).
rat-race (n.): also rat race, 1934 in reference to aviation training, from rat (n.) + race (n.1). Rat-run is from 1870 in a literal sense.
A rat race is ... a simple game of "follow the leader" in fighter planes. The leader does everything he can think of -- Immelmanns, loops, snap rolls, and turns, always turns, tighter and tighter. ["Popular Science," May 1941]
Meaning "competitive struggle" is from 1945.
rat-tail (n.): also rat's-tail, used since 16c. of conditions or devices held to resemble a rat's long, hairless tail in some sense, including "lank lock of hair" (1810); "end of a rope" (1867); from rat (n.) + tail (n.1).
rat-trap (n.): late 15c., from rat (n.) + trap (n.).
ratable (adj.): also rateable, c. 1500, from rate (v.2) + -able.
ratafia (n.): liqueur flavored with kernels of cherries, apricots, etc., 1690s, from French ratafia (17c.), of unknown origin; perhaps ultimately from the same source as arrack.
ratatouille (n.): 1877, from French ratatouille (19c.), first element uncertain, second element evidently touiller "to stir up," ultimately from Latin tudes "hammer" [Gamillscheg].
ratbag (n.): also rat-bag, "unpleasant person," 1937, from rat (n.) + bag (n.).
ratchet (n.): 1650s, rochet, from French rochet "bobbin, spindle," from Italian rocchetto "spool, ratchet," diminutive of rocca "distaff," possibly from a Germanic source (compare Old High German rocko "distaff," Old Norse rokkr), from Proto-Germanic *rukka-, from PIE root *ruk- "fabric, spun yarn." Compare rocket (n.2). Current spelling in English dates from 1721, influenced by synonymous ratch, which perhaps is borrowed from German Rätsche "ratchet."
ratchet (v.): 1852, from ratchet (n.). Transferred sense attested by 1977. Related: Ratcheted; ratcheting.
rate (n.): "estimated value or worth," early 15c., from Old French rate "price, value" and directly 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" (properly 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, accuse, find fault with," 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," mid-15c., from rate (n.). Intransitive sense of "have a certain value, rank, or standing" is from 1809; specifically as "have high value" from 1928. Related: Rated; rating.
rather (adv.): Old English hraþor "more quickly, earlier, sooner," also "more readily," comparative of hraþe, hræþe "quickly, hastily, promptly, readily, immediately," which is related to hræð "quick, nimble, prompt, ready," from Proto-Germanic *khratha- (source also of Old Norse hraðr, Old High German hrad), from PIE *kret- "to shake." The base form rathe was obsolete by 18c. except in poetry (Tennyson); superlative rathest fell from use by 17c. Meaning "more willingly" is recorded from c. 1300; sense of "more truly" is attested from late 14c.
The rather lambes bene starved with cold
[Spenser, "The Shepheardes Calender" (Februarie), 1579]
rathskeller (n.): 1900, from German ratskeller, earlier rathskeller, "a cellar in a German town hall in which beer is sold," from rat "council" (see rede (n.)) + keller "cellar" (see cellar (n.)). The German -h- inserted to avoid association with the word for "rat."
ratification (n.): mid-15c., from Middle French ratification (14c.) and directly from Medieval Latin ratificationem (nominative ratificatio), noun of action from past participle stem of ratificare (see ratify).
ratify (v.): mid-14c., from Old French ratifier (13c.), from Medieval Latin ratificare "confirm, approve," literally "fix by reckoning," from Latin ratus "fixed by calculation; determined; approved; certain, sure; valid" (past participle adjective from reri "to reckon, think;" see reason (v.)) + root of facere "to make" (see factitious). Related: Ratified; ratifying.
rating (n.): 1530s, "a fixing of rates," verbal noun from rate (v.2). Meaning "a classification according to rates" is from 1764. 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.
ratio (n.): 1630s, "reason, rationale," from Latin ratio "reckoning, numbering, calculation; business affair, procedure," also "reason, reasoning, judgment, understanding," from rat-, past participle stem of reri "to reckon, calculate," also "think" (see reason (n.)). Mathematical sense "relationship between two numbers" is attested from 1650s.
ratiocinate (v.): "to reason," 1640s, from Latin ratiocinatus, past participle of ratiocinare (see ratiocination). Related: Ratiocinant.
ratiocination (n.): "process of reasoning," 1520s, from Latin ratiocinationem (nominative ratiocinatio) "a reasoning, calm reasoning," from past participle stem of ratiocinare "to calculate, deliberate," from ratio (see ratio) + -cinari, which probably is related to conari "to try" (see conation).
Most writers make ratiocination synonymous with reasoning. J.S. Mill and others hold that the word is usually limited to necessary reasoning. [Century Dictionary]
ration (n.): 1550, "reasoning," later, "relation of one number to another" (1660s), then "fixed allowance of food" (1702, often rations, from French ration in this sense), from Latin rationem (nominative ratio) "a reckoning, calculation, proportion" (see ratio). The military pronunciation (rhymes with fashion) took over from the preferred civilian pronunciation (rhymes with nation) during World War I.
ration (v.): "put (someone) on a fixed allowance," 1859, from ration (n.); sense of "apportion in fixed amounts" is from 1870. Related: Rationed; rationing.
rational (adj.): late 14c., "pertaining to reason;" mid-15c., "endowed with reason," from Old French racionel and directly from Latin rationalis "of or belonging to reason, reasonable," from ratio (genitive rationis) "reckoning, calculation, reason" (see ratio).
rationale (n.): 1650s, "exposition of principles," from Late Latin rationale, noun use of neuter of Latin rationalis "of reason" (see rational). Hence, "fundamental reason" (1680s).
rationalise (v.): chiefly British English spelling of rationalize. For suffix, see -ize. Related: Rationalised; rationalising; rationalisation.
rationalise (v.): chiefly British English spelling of rationalize; see -ize. Related: Rationalised; rationalising; rationalisation.
rationalist (n.): "one who follows reason and not authority in thought or speculation," originally especially "physician whose treatment is based on reason," 1620s, from rational + -ist. Applied to a philosophical doctrine 1640s. Related: Rationalism.
rationality (n.): 1620s, "quality of having reason;" 1650s, "fact of being agreeable to reason," from French rationalité, from Late Latin rationalitas "reasonableness, rationality" (also source of Spanish racionalidad, Italian razionalita), from Latin rationalis (see rational).
rationalization (n.): 1825, "a rendering rational," from rationalize + -ation. Psychological use is from 1908.
Of the three works now on our table, the two which we have placed first have these laudable objects in view; an improvement on the former versions of the Psalms as compositions, and the rationalization, if we may so speak, of our Church psalmody. ["The British Critic," London, Jan.-June 1825]
rationalize (v.): 1767, "explain in a rational way, make conformable to reason," from rational + -ize. In the psychological sense of "to give an explanation that conceals true motives" it dates from 1922. Related: Rationalized; rationalizing.
rationing (n.): "restriction to limited allotments," 1865, verbal noun from ration (v.). Specifically of restrictions during wartime from 1917, from conditions in England during World War I.
ratline (n.): "thin rope," especially as used on sailing ships, late 15c., originally ratling, of unknown origin; spelling ratline attested from 1773, by influence of line (n.).
rats (interj.): expressing incredulity, disappointment, etc., 1886, from rat (n.).
ratsbane (n.): "rat poison, arsenic," 1520s; see rat (n.) + bane.
rattan: 1650s, from Malay rotan, from raut "to trim, strip, peel, pare."
rattle (v.): c. 1300 (intransitive), "To make a quick sharp noise with frequent repetitions and collisions of bodies not very sonorous: when bodies are sonorous, it is called jingling" [Johnson]. Perhaps in Old English but not recorded; if not, from Middle Dutch ratelen, probably of imitative origin (compare German rasseln "to rattle," Greek kradao "I rattle"). Sense of "utter smartly and rapidly" is late 14c. Meaning "to go along loosely and noisily" is from 1550s. Transitive sense is late 14c.; figurative sense of "fluster" is first recorded 1869. Related: Rattled; rattling.
rattle (n.): c. 1500, "rapid succession of short, sharp sounds," from rattle (v.). As a child's toy, recorded from 1510s. As a sound made in the throat (especially of one near death) from 1752.
rattler (n.): mid-15c., "one who talks overmuch," agent noun from rattle (v.). From c. 1300 as a surname. As short for rattlesnake, 1827.
rattlesnake (n.): 1620s, from rattle + snake (n.).
rattletrap: 1766, originally a noun, "knick-knacks, trifles, odds and ends," from rattle (adj.) + trap, perhaps in the sense in traps (n.2). Hence, "a shabby, rattling object," especially a rickety coach or other vehicle (1822). The adjectival sense of "rickety" is recorded from 1834.
ratty (adj.): 1856, "resembling a rat;" 1865, "full of rats;" 1867, "wretched, miserable, shabby," from rat (n.) + -y (2).
raucous (adj.): 1769, from Latin raucus "hoarse" (also source of French rauque, Spanish ronco, Italian rauco), related to ravus "hoarse," from PIE echoic base *reu- "make hoarse cries" (source also of Sanskrit rayati "barks," ravati "roars;" Greek oryesthai "to howl, roar;" Latin racco "a roar;" Old Church Slavonic rjevo "I roar;" Lithuanian rekti "roar;" Old English rarian "to wail, bellow"). Middle English had rauc in the same sense, from the same source.
raunch (n.): 1963 (in "Billboard," describing lead guitar on surf music tracks), back-formation from raunchy. There was a singing group in U.S. c. 1960 called the Raunch Hands.