Online Etymology Dictionary

Rastus: also 'Rastus, masc. proper name, short for Erastus. As a stereotypical name of an American slave or derogatory for "black man," 1881, probably from use in stories of Joel Chandler Harris.
rat (n.): late Old English ræt "rat," of uncertain origin. Similar words are found in Celtic (Gaelic radan), Romanic (Italian ratto, Spanish rata, French rat) and Germanic (Old Saxon ratta; Dutch rat; German Ratte, dialectal Ratz; Swedish råtta, Danish rotte) languages, but connection is uncertain and origin unknown. In all this it is very much like cat.

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 1540s. _____-rat, "person who frequents _____" (in earliest reference dock-rat) is from 1864.
rat (v.): 1812, "to desert one's party; 1864 as "to catch rats;" 1921 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, "competitive struggle," 1934, from rat (n.) + race (n.1). Rat-run is from 1870 in a literal sense.
rat-tail (n.): also rat's-tail, used since 16c. of conditions or devices he;d 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.).
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 (v.): 1852, from ratchet (n.). Transferred sense attested by 1977. Related: Ratcheted; ratcheting.
ratchet (n.): 1650s, rochet, 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 *ruk- "fabric, spun yarn." Cf. rocket (n.2). Current spelling in English dates from 1721, influenced by synonymous ratch, which perhaps is borrowed from German Rätsche "ratchet."
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" (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, 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- (cf. 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 read (v.)) + 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), perhaps in Old English but not recorded; if not, from Middle Dutch ratelen, probably of imitative origin (cf. 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. 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).