rat (n.) Look up rat at Dictionary.com
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 from PIE root *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 further etymology.
RATS. Of these there are the following kinds: a black rat and a grey rat, a py-rat and a cu-rat. ["Dictionary of the Vulgar Tongue," Grose, 1788]
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; "to be put on the watch by suspicion as the cat by the scent of a rat; to suspect danger" [Johnson]. _____-rat, "person who frequents _____" (in earliest reference dock-rat) is from 1864.
rat fink (n.) Look up rat fink at Dictionary.com
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 Look up rat-a-tat at Dictionary.com
1680s, echoic, originally of a cooper hammering tubs.
rat-catcher (n.) Look up rat-catcher at Dictionary.com
1590s, from rat (n.) + catcher.
rat-hole (n.) Look up rat-hole at Dictionary.com
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.) Look up rat-pack at Dictionary.com
"juvenile gang," 1951, from rat (n.) + pack (n.). In reference to a Hollywood circle around Frank Sinatra, from 1958.
rat-poison (n.) Look up rat-poison at Dictionary.com
"arsenic," 1799, from rat (n.) + poison (n.).
rat-race (n.) Look up rat-race at Dictionary.com
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.) Look up rat-tail at Dictionary.com
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.) Look up rat-trap at Dictionary.com
late 15c., from rat (n.) + trap (n.).
ratable (adj.) Look up ratable at Dictionary.com
also rateable, c. 1500, from rate (v.2) + -able.
ratafia (n.) Look up ratafia at Dictionary.com
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.) Look up ratatouille at Dictionary.com
1877, from French ratatouille (19c.), first element uncertain, second element evidently touiller "to stir up," ultimately from Latin tudes "hammer" [Gamillscheg].
ratbag (n.) Look up ratbag at Dictionary.com
also rat-bag, "unpleasant person," 1937, from rat (n.) + bag (n.).
ratchet (n.) Look up ratchet at Dictionary.com
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.) Look up ratchet at Dictionary.com
1852, from ratchet (n.). Transferred sense attested by 1977. Related: Ratcheted; ratcheting.
rate (v.2) Look up rate at Dictionary.com
"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.
rate (n.) Look up rate at Dictionary.com
"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" (from PIE root *re- "to reason, count"). 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) Look up rate at Dictionary.com
"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," from re- "repeatedly" (see re-) + putare "to judge, suppose, believe, suspect," originally "to clean, trim, prune" (from PIE root *pau- (2) "to cut, strike, stamp"). Related: Rated; rating.
rather (adv.) Look up rather at Dictionary.com
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.) Look up rathskeller at Dictionary.com
1900, from German ratskeller, earlier rathskeller, "a cellar in a German town hall in which beer is sold," from rat "council" (from Proto-Germanic *redaz, from suffixed form of PIE root *re- "to reason, count") + keller "cellar" (see cellar (n.)). The German -h- inserted to avoid association with the word for "rat."
ratification (n.) Look up ratification at Dictionary.com
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.) Look up ratify at Dictionary.com
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;" from PIE root *re- "to reason, count") + combining form of facere "to make" (from PIE root *dhe- "to set, put"). Related: Ratified; ratifying.
rating (n.) Look up rating at Dictionary.com
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.) Look up ratio at Dictionary.com
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" (from PIE root *re- "to reason, count"). Mathematical sense "relationship between two numbers" is attested from 1650s.
ratiocinate (v.) Look up ratiocinate at Dictionary.com
"to reason," 1640s, from Latin ratiocinatus, past participle of ratiocinare (see ratiocination). Related: Ratiocinant.
ratiocination (n.) Look up ratiocination at Dictionary.com
"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 (v.) Look up ration at Dictionary.com
"put (someone) on a fixed allowance," 1859, from ration (n.); sense of "apportion in fixed amounts" is from 1870. Related: Rationed; rationing.
ration (n.) Look up ration at Dictionary.com
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) "reckoning, numbering, calculation; business affair, procedure," also "reason, reasoning, judgment, understanding," from rat-, past participle stem of reri "to reckon, calculate," also "think" (from PIE root *re- "to reason, count"). The military pronunciation (rhymes with fashion) took over from the preferred civilian pronunciation (rhymes with nation) during World War I.
rational (adj.) Look up rational at Dictionary.com
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.) Look up rationale at Dictionary.com
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.) Look up rationalise at Dictionary.com
chiefly British English spelling of rationalize. For suffix, see -ize. Related: Rationalised; rationalising; rationalisation.
rationalise (v.) Look up rationalise at Dictionary.com
chiefly British English spelling of rationalize; see -ize. Related: Rationalised; rationalising; rationalisation.
rationalist (n.) Look up rationalist at Dictionary.com
"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.) Look up rationality at Dictionary.com
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.) Look up rationalization at Dictionary.com
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.) Look up rationalize at Dictionary.com
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.) Look up rationing at Dictionary.com
"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.) Look up ratline at Dictionary.com
"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.) Look up rats at Dictionary.com
expressing incredulity, disappointment, etc., 1886, from rat (n.).
ratsbane (n.) Look up ratsbane at Dictionary.com
"rat poison, arsenic," 1520s; see rat (n.) + bane.
rattan Look up rattan at Dictionary.com
1650s, from Malay (Austronesian) rotan, from raut "to trim, strip, peel, pare."
rattle (v.) Look up rattle at Dictionary.com
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.) Look up rattle at Dictionary.com
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.) Look up rattler at Dictionary.com
mid-15c., "one who talks overmuch," agent noun from rattle (v.). From c. 1300 as a surname. As short for rattlesnake, 1827.
rattlesnake (n.) Look up rattlesnake at Dictionary.com
1620s, from rattle + snake (n.).
rattletrap Look up rattletrap at Dictionary.com
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.) Look up ratty at Dictionary.com
1856, "resembling a rat;" 1865, "full of rats;" 1867, "wretched, miserable, shabby," from rat (n.) + -y (2).
raucous (adj.) Look up raucous at Dictionary.com
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.) Look up raunch at Dictionary.com
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.