Online Etymology Dictionary

racquet (n.): "handled hitting device used in tennis, etc.," c. 1500, probably originally "tennis-like game played with open hand" (late 14c.), from Middle French rachette, requette (Modern French raquette) "racket for hitting; palm of the hand," perhaps via Italian racchetta or Spanish raqueta, both often said to be from Arabic rahat, a form of raha "palm of the hand," but this has been doubted. Compare French jeu de paume "tennis," literally "play with the palm of the hand," and compare tennis).
racquetball (n.): 1972, from racquet + ball (n.1).
racy (adj.): 1650s, "having a characteristic taste" (of wines, fruits, etc.), from race (n.2) in its older sense of "flavor" or in the sense "class of wines" + -y (2); meaning "having a quality of vigor" (1660s) led to that of "improper, risqué," first recorded 1901, probably reinforced by phrase racy of the soil "earthy" (1870). Related: Racily; raciness.
rad: "x-ray dose unit," 1918, shortened form of radiation (q.v.). As shortened form of radical (n.), it is attested in political slang from 1820. Teen slang sense of "extraordinary, wonderful" is from late 1970s (see radical (adj.)).
radar (n.): "electronic system for locating objects by means of radio waves," 1941, acronym (more or less) from radio detecting and ranging. The U.S. choice, it won out over British radiolocation. Figurative from 1950.
radial (adj.): c. 1400, "of or like a ray or radius," from Medieval Latin radialis, from Latin radius "shaft, rod; spoke of a wheel; beam of light" (see radius). As a noun, a type of tire, attested from 1965, short for radial-ply (tire). Related: Radially.
radian (n.): "angle subtended at the center of a circle by an arc equal in length to the radius," 1879, from radius.
radiance (n.): c. 1600, "brilliant light," from radiant or else from Medieval Latin radiantia "brightness," from radiare "to beam, shine" (see radiation). Figurative use from 1761. Related: Radiancy.
radiant (adj.): mid-15c., from Middle French radiant and directly from Latin radiantem (nominative radians) "beaming, shining," present participle of radiare "to beam, shine" (see radiation). Of beauty, etc., first attested c. 1500. Related: Radiantly.
radiant (n.): "point or object from which light radiates," 1727; see radiant (adj.). In astronomy, of meteor showers, from 1864.
radiate (adj.): "having rays, furnished with rays, shining," 1660s, from Latin radiatus (see radiate (v.)).
radiate (v.): 1610s, "spread in all directions from a point," from Latin radiatus, past participle of radiare "to beam, shine, gleam; make beaming," from radius "beam of light; spoke of a wheel" (see radius). Meaning "be radiant, give off rays (of light or heat)" is from 1704. Related: Radiated; radiates; radiating.
radiation (n.): mid-15c., "act or process of radiating," from Middle French radiation and directly from Latin radiationem (nominative radiatio) "a shining, radiation," noun of action from past participle stem of radiare "to beam, shine, gleam; make beaming," from radius "beam of light; spoke of a wheel" (see radius). Meaning "rays or beams emitted" is from 1560s. Meaning "divergence from a center" is 1650s.
radiative (adj.): "having a tendency to radiate," 1820, from radiate (v.) + -ive. Related: Radiativity.
radiator (n.): 1836, "any thing that radiates," agent noun in Latin form from radiate. Meaning "heater" is from 1851; sense of "cooling device in internal combustion engine" is 1900.
radical (adj.): late 14c., in a medieval philosophical sense, from Late Latin radicalis "of or having roots," from Latin radix (genitive radicis) "root" (see radish). Meaning "going to the origin, essential" is from 1650s. Radical sign in mathematics is from 1680s.

Political sense of "reformist" (via notion of "change from the roots") is first recorded 1802 (n.), 1817 (adj.), of the extreme section of the British Liberal party (radical reform had been a current phrase since 1786); meaning "unconventional" is from 1921. U.S. youth slang use is from 1983, from 1970s surfer slang meaning "at the limits of control." Radical chic is attested from 1970; popularized, if not coined, by Tom Wolfe. Radical empiricism coined 1897 by William James (see empiricism).
radical (n.): 1630s, "root part of a word, from radical (adj.) Political sense from 1802; chemical sense from 1816.
radicalism (n.): 1819 in the political sense, from radical (adj.) + -ism.
radicality (n.): 1640s, from radical (adj.) + -ity.
radicalize (v.): 1820, from radical (adj.) + -ize. Related: Radicalized; radicalizing.
radically (adv.): c. 1600, "thoroughly;" 1620s with reference to roots and origins, from radical (adj.) + -ly (2).
radicand (n.): the number under a radical sign, from Latin radicandus, gerundive of radicare (see radicant).
radicant (adj.): "bringing forth roots," 1735, from Latin radicantem (nominative radicans), present participle of radicare, from radix "root" (see radish).
radicle (n.): 1670s, in botany, from Latin radicula, diminutive of radix (see radish).
radicular (adj.): "pertaining to roots," 1830, from Modern Latin radicula, diminutive of Latin radix (see radish) + -ar.
radio (n.): "wireless transmission of voice signals with radio waves," 1907, abstracted from earlier combinations such as radio-receiver (1903), radiophone (1881), radio-telegraphy (1898), from radio- as a comb. form of Latin radius "beam." Use for "radio receiver" is first attested 1913; sense of "sound broadcasting as a medium" is from 1913.
It is not a dream, but a probability that the radio will demolish blocs, cut the strings of red tape, actuate the voice "back home," dismantle politics and entrench the nation's executive in a position of power unlike that within the grasp of any executive in the world's history. ["The Reading Eagle," Reading, Pa., U.S.A., March 16, 1924]
In U.S., stations were broadcasting news and music by late 1920, but the new medium caught on nationwide as a fad in the winter of 1921-22; as late as July 1921 the "New York Times" had called it wireless telephony, and wireless remained more widespread until World War II, when military preference for radio turned the tables. As an adjective by 1912, "by radio transmission;" meaning "controlled by radio" from 1974. Radio _______ "radio station or service from _______" is recorded from 1920. A radio shack (1946) was a small building housing radio equipment.
radio (v.): 1916, from radio (n.). Related: Radioed; radioing.
radio-: word-forming element meaning 1. "ray, ray-like" (see radius); 2. "radial, radially" (see radial (adj.)); 3. "by means of radiant energy" (see radiate (v.)); 4. "radioactive" (see radioactive); 5. "by radio" (see radio (n.)).
radio-carbon (n.): "Carbon-14," 1940, from radio-, comb. form of radioactive, + carbon. Radio-carbon dating is attested from 1949.
radio-telephone (n.): 1900, from radio (n.) + telephone (n.).
radioactive (adj.): 1898, from French radio-actif, coined by Pierre and Marie Curie from radio-, comb. form of Latin radius (see radiation) + actif "active" (see active).
radioactivity (n.): 1899, from French radioactivité, coined 1898 by the Curies; see radioactive.
radiocast (n.): 1924, from radio (n.) + ending from broadcast.
radiography (n.): 1896, from radiograph (1880), originally a device to measure sunshine; from radio-, comb. form of radiation, + -graph. As a type of image-making device, from 1896.
radioisotope (n.): 1946, from radio-, comb. form of radiation, + isotope.
radiolarian (n.): 1862, from Radiolaria, Modern Latin classification name, from Latin radiolus, diminutive of radius (q.v.).
radiology (n.): 1900, "medical use of X-rays," later extended to "scientific study of radiation," from radio-, comb. form of radiation, + Greek-based scientific suffix -ology. Related: Radiological.
radiometric (adj.): 1877, from radiometer "instrument to transform radiant energy into mechanical work" (1875), radiometry, from radio-, here indicating "radiant energy," + -metric. Radiometric dating attested from 1906.
radioscopy (n.): 1896, from radio- + -scopy.
radiotherapy (n.): 1903, from radio- + therapy.
radish (n.): late Old English rædic "radish," from Latin radicem (nominative radix) "root, radish," from PIE root *wrad- "twig, root" (source also of Greek rhiza, Lesbian brisda "root;" Greek hradamnos "branch;" Gothic waurts, Old English wyrt; Welsh gwridd, Old Irish fren "root"). Spelling perhaps influenced by Old French radise, variant of radice, from Vulgar Latin *radicina, from radicem.
radium (n.): radioactive metallic element, 1899, from French radium, named 1898 after identification by Marie Curie and her husband, formed in Modern Latin from Latin radius "ray" (see radius). With metallic element ending -ium. So called for its power of emitting energy in the form of rays.
radius (n.): 1590s, "cross-shaft," from Latin radius "staff, stake, rod; spoke of a wheel; ray of light, beam of light; radius of a circle," of unknown origin. Perhaps related to radix "root," but Tucker suggests connection to Sanskrit vardhate "rises, makes grow," via root *neredh- "rise, out, extend forth;" or else Greek ardis "sharp point."

The geometric sense first recorded 1610s. Plural is radii. Meaning "circular area of defined distance around some place" is attested from 1953. Meaning "shorter bone of the forearm" is from 1610s in English (the Latin word had been used thus by the Romans).
Radnor: place in eastern Wales, the name is Old English, literally "at the red bank," from Old English read (dative singular readan; see red (n.1) + ofer "bank, slope."
radon (n.): heaviest gaseous element, 1918, from German Radon, from radium (q.v.) + -on suffix of inert gases. The element was identified in radioactive decay of radium. Alternative name niton (from Latin nitens "shining") gained currency in France and Germany.
radula (n.): surgical instrument, 1753, from Latin radula "scraper, scraping iron," from radere "to scrape" (see raze). Related: Radular.
raffia (n.): fiber-yielding tree of Madagascar, 1729, rofia, from Malagasy rafia. Modern form is attested from 1882; also raphia (1866).
raffish (adj.): "disreputable, vulgar," 1795, from raff "people," usually of a lower sort (1670s), probably from rif and raf (mid-14c.) "everyone," from Middle English raf, raffe "one and all, everybody" (see riffraff). Related: Raffishly; raffishness.
raffle (n.): late 14c., "dice game," from Old French rafle "dice game," also "plundering," perhaps from a Germanic source (compare Middle Dutch raffel "dice game," Old Frisian hreppa "to move," Old Norse hreppa "to reach, get," Swedish rafs "rubbish," Old High German raspon "to scrape together, snatch up in haste," German raffen "to snatch away, sweep off"), from Proto-Germanic *khrap- "to pluck out, snatch off." The notion would be "to sweep up (the stakes), to snatch (the winnings)." Dietz connects the French word with the Germanic root, but OED is against this. Meaning "sale of chances" first recorded 1766.
raffle (v.): "dispose of by raffle," 1851, from raffle (n.). Related: Raffled; raffling.