Online Etymology Dictionary

reaper (n.): Old English ripere, agent noun from reap (v.). As the name of a personification of death, from 1839.
rear (n.): "hindmost part," c. 1600, abstracted from rerewarde "rear guard, hindmost part of an army or fleet" (mid-14c.), from Anglo-French rerewarde, Old French rieregarde, from Old French adverb riere "behind" (from Latin retro "back, behind;" see retro-) + Old French garde (see guard (n.)). Or the word may be a shortened form of arrear (see arrears).

As a euphemism for "buttocks" it is attested from 1796. Rear admiral is first attested 1580s, apparently so called from ranking "behind" an admiral proper. Rear-view (mirror) is recorded from 1926.
rear (v.1): Old English ræran "to raise, build up, create, set on end; arouse, excite, stir up," from Proto-Germanic *raizijanau "to raise," causative of *risanan "to rise" (see rise (v.)). The second -r- is by rhotacism. Meaning "bring into being, bring up" (as a child) is recorded from early 15c.; that of "raise up on the hind legs" is first recorded late 14c. Related: Reared; rearing.
rear (adj.): c. 1300, from Old French rere (see rear (n.)).
rear (v.2): "attack in the rear," 17c., from rear (n.).
rear-end (n.): "buttocks," 1937, from rear (adj.) + end (n.). As a verb, "to collide (with another vehicle) from behind," from 1976. Related: Rear-ended; rear-ending.
rearward (adv.): 1590s, from rear (adj.) + -ward.
reason (n.): c. 1200, "intellectual faculty that adopts actions to ends," also "statement in an argument, statement of explanation or justification," from Anglo-French resoun, Old French raison "course; matter; subject; language, speech; thought, opinion," from Latin rationem (nominative ratio) "reckoning, understanding, motive, cause," from ratus, past participle of reri "to reckon, think," from PIE root *re(i)- "to reason, count" (source of Old English rædan "to advise;" see read (v.)).

Meaning "sanity; degree of intelligence that distinguishes men from brutes" is recorded from late 13c. Sense of "grounds for action, motive, cause of an event" is from c. 1300. Middle English sense of "meaning, signification" (early 14c.) is in the phrase rhyme or reason. Phrase it stands to reason is from 1630s. Age of Reason "the Enlightenment" is first recorded 1794, as the title of Tom Paine's book.
reason (v.): early 14c., resunmen, "to question (someone)," also "to challenge," from Old French raisoner "speak, discuss; argue; address; speak to," from Late Latin rationare "to discourse," from ratio (see reason (n.)). Intransitive sense of "to think in a logical manner" is from 1590s; transitive sense of "employ reasoning (with someone)" is from 1847. Related: Reasoned; reasoning.
reasonable (adj.): c. 1300, "having sound judgment, sane, rational," from Old French raisonable, from Latin rationabilis, from ratio (see reason (n.)).
What the majority of people consider to be 'reasonable' is that about which there is agreement, if not among all, at least among a substantial number of people; 'reasonable' for most people, has nothing to do with reason, but with consensus. [Erich Fromm, "The Heart of Man," 1968]
Meaning "moderate in price" is recorded from 1660s. Related: Reasonably.
reasoning (n.): late 14c., "exercise of the power of reason; act or process of thinking logically;" also "an instance of this;" verbal noun from reason (v.).
reassurance (n.): also re-assurance, 1610s, from reassure + -ance.
reassure (v.): "restore (someone) to confidence," 1590s, from re- "back, again" + assure. Related: Reassured; reassuring.
reattach (v.): also re-attach, c. 1600 in law; 1813 in a literal sense, from re- + attach. Related: Reattached; reattaching.
reave (v.): Old English reafian "to rob (something from someone), plunder, pillage," from Proto-Germanic *raubjon (source also of Old Frisian ravia, Middle Dutch roven, Dutch rooven, Old High German roubon, German rauben), from PIE *reup- "to snatch" (see rip (v.)). Related: Reaved; reaving.
reaver (n.): Old English reafere "plundering forager," agent noun from reafian (see reave (v.)). Similar formation in Old Frisian ravere, Middle Dutch rover, Dutch roover, Old High German roubari, German Räuber.
reb (n.): abbreviation of rebel (n.), 1862, in U.S. Civil War context.
rebar (n.): also re-bar, "steel reinforcing rod in concrete," 1961, from re(inforced) bar.
rebarbative (adj.): "repellent, unattractive," 1885, from French rébarbatif (14c.), from barbe "beard," from Latin barba (see barb (n.)).
rebate (v.): late 14c., "to reduce;" early 15c., "to deduct, subtract," from Old French rabattre "beat down, drive back," also "deduct," from re- "repeatedly" (see re-) + abattre "beat down" (see abate). Meaning "to pay back (a sum) as a rebate" is from 1957. Related: Rebated; rebating.
rebate (n.): 1650s, from rebate (v.).
rebbe (n.): 1881, from Yiddish, from Hebrew rabbi (see rabbi).
rebec (n.): medieval stringed musical instrument, early 15c., from Middle French rebec (15c.), an unexplained alteration (perhaps somehow influenced by bec "beak") of Old French ribabe (13c.), ultimately from Arabic rebab. Compare Old Provençal rebec, Italian ribeca. It has three strings and is played with a bow.
Rebecca: fem. proper name, biblical wife of Isaac, mother of Jacob and Esau, from Late Latin Rebecca, from Greek Rhebekka, from Hebrew Ribhqeh, literally "connection" (compare ribhqah "team"), from Semitic base r-b-q "to tie, couple, join" (compare Arabic rabaqa "he tied fast"). Rebekah, the form of the name in Authorized Version, was taken as the name of a society of women (founded 1851 in Indiana, U.S.) as a complement to the Odd Fellows.
rebel (adj.): c. 1300, from Old French rebelle "stubborn, obstinate, rebellious" (12c.) and directly from Latin rebellis "insurgent, rebellious," from rebellare "to rebel, revolt," from re- "opposite, against," or perhaps "again" (see re-) + bellare "wage war," from bellum "war."
rebel (v.): mid-14c., from Old French rebeller (14c.), from Latin rebellare "to revolt" (see rebel (adj.)). Related: Rebelled; rebelling.
rebel (n.): "person who makes war on his country for political motives," mid-14c., from rebel (adj.). Meaning "supporter of the American cause in the War of Independence" is from 1775; sense of "supporter of the Southern cause in the American Civil War" is attested from April 15, 1861. Rebel yell in an American Civil War context attested from 1862, but the thing itself is older and was said to have been picked up by (then) southwestern men in their periodic wars against the Indians.
The Southern troops, when charging or to express their delight, always yell in a manner peculiar to themselves. The Yankee cheer is more like ours; but the Confederate officers declare that the rebel yell has a particular merit, and always produces a salutary and useful effect upon their adversaries. A corps is sometimes spoken of as a 'good yelling regiment.' [A.J.L. Fremantle, "The Battle of Gettysburg and the Campaign in Pennsylvania," in "Blackwood's Edinburgh Magazine," Sept. 1863]
rebellion (n.): "war waged against a government by some portion of its subjects," mid-14c., from Old French rebellion (14c.) and directly from Latin rebellionem (nominative rebellio) "rebellion, revolt; renewal of war," from rebellis (see rebel (adj.)).
rebellious (adj.): early 15c., from Latin rebellis (see rebel (adj.)) + -ous. Related: Rebelliously; rebelliousness.
rebirth (n.): 1812, "reincarnation;" 1833, "renewed life or activity," from re- + birth (n.).
reboot (v.): 1981, from re- + boot (v.) in the computer sense. Related: Rebooted; rebooting.
rebop (n.): see bebop.
reborn (adj.): 1590s, from re- "back, again" + born.
reborrow (v.): 1630s, from re- "back, again" + borrow. Related: Reborrowed; reborrowing.
rebound (v.): late 14c., "to spring, leap," also "return to afflict" (early 15c.), from Old French rebondir "leap back, resound; repulse, push back," from re- "back" (see re-) + bondir "leap, bound" (see bound (v.)). Sense of "to spring back from force of impact" is recorded from late 14c. Sports use probably first in tennis; basketball sense is attested from 1914. Related: Rebounded; rebounding.
rebound (n.): 1520s, in reference to a ball, from rebound (v.). Sense in basketball from 1920 (from 1917 in ice hockey). Meaning "period of reaction or renewed activity after disturbance" is from 1570s.
rebroadcast (v.): also re-broadcast, 1923, from re- + broadcast (v.). Related: Rebroadcasting.
rebuff (v.): 1580s, from obsolete French rebuffer "to check, snub," from Italian ribuffare "to check, chide, snide," from ribuffo "a snub," from ri- "back" (from Latin re-, see re-) + buffo "a puff," of imitative origin (compare buffet (v.)). Related: Rebuffed; rebuffing.
rebuff (n.): 1610s, from rebuff (v.), or from Middle French rebuffe or Italian ribuffo.
rebuild (v.): c. 1600 (implied in rebuilding), from re- "back, again" + build (v.). Related: Rebuilt.
rebuke (v.): early 14c., "to reprimand, reprove; chide, scold," from Anglo-French rebuker "to repel, beat back," Old French rebuchier, from re- "back" (see re-) + buschier "to strike, chop wood," from busche (French bûche) "wood," from Proto-Germanic *busk- (see bush (n.)). Related: Rebuked; rebuking.
rebuke (n.): early 15c., "a reproof, reprimand," from rebuke (v.).
rebus (n.): c. 1600, from Latin rebus "by means of objects," ablative plural of res "thing, object." According to French sources, principally from the phrase de rebus quæ geruntur "of things which are going on," in reference to the satirical pieces composed by Picardy clerks at carnivals, subtle satires of current events using pictures to suggest words, phrases or things. Or it may be from the representations being non verbis sed rebus "not by words, but by things." In either case from Latin res "thing."
rebut (v.): c. 1300, "to thrust back," from Old French reboter, rebuter "to thrust back," from re- "back" (see re-) + boter "to thrust, hit" (see butt (v.)). Legalese sense of "try to disprove, refute by evidence or argument" is from 1817. Related: Rebutted; rebutting.
rebuttal (n.): 1793, from rebut + -al (2). Earlier were rebutment (1590s) and rebutter (1530s, in law).
rec (n.): 1929 as a shortening of recreation.
recalcitrance (n.): 1845, from French récalcitrance or from recalcitrant + -ance.
recalcitrant (adj.): 1823, from French récalcitrant, literally "kicking back" (17c.-18c.), past participle of recalcitrare "to kick back; be inaccessible," from re- "back" (see re-) + Latin calcitrare "to kick," from calx (genitive calcis) "heel" (see calcaneus). Used from 1797 as a French word in English.
recalcitrate (v.): "to kick out," 1620s, from Latin recalcitratus, past participle of recalcitrare (see recalcitrant). Sense of "resist obstinately" is from 1759. Related: Recalcitrated; recalcitrating.
recalibrate (v.): 1883, from re- + calibrate. Related: Recalibrated; recalibrating.