Online Etymology Dictionary

re-enforce (v.): also reenforce, 1580s, "to give fresh strength to," from re- "back, again" + enforce (v.). Originally of persons or military units; of buildings, structures, etc., attested from 1883. Related: Re-enforced; re-enforcing.
re-enlist (v.): also reenlist, 1828, from re- "back, again" + enlist. Related: Re-enlisted; re-enlisting.
re-entry (n.): also reentry, mid-15c., "act of entering again," from re- "again" + entren (see enter), probably on model of Middle French rentrer (12c.). Specifically of spacecraft from 1948.
re-establish (v.): also reestablish, late 15c.; from re- + establish. Related: Re-established; re-establishing.
re-evaluate (v.): also reevaluate, 1903, from re- + evaluate. Related: Reevaluated; reevaluating.
re-evaluation (n.): also reevaluation, 1905; noun of action from re-evaluate.
re-examine (v.): also reexamine, 1590s, from re- + examine. Related: Re-examined; re-examining.
re-up (v.): "to re-enlist," 1906, U.S. armed forces slang, from re- "back, again" + up "enlist." Related: Re-upped; re-upping.
reabsorption (n.): 1755, from re- + absorption.
reach (v.): Old English ræcan "to extend, hold forth," also "to succeed in touching," from West Germanic *raikjan "stretch out the hand" (cf. Old Frisian reka, Middle Dutch reiken), from Proto-Germanic *raikijanau, perhaps from PIE root *reig- "to stretch out" (cf. Sanskrit rjyati "he stretches himself," riag "torture" (by racking); Greek oregein "to reach, extend;" Lithuanian raizius "to stretch oneself;" Old Irish rigim "I stretch"), related to base *reg- "to rule, to lead straight, to put right" (see regal).

Shakespeare uses the now-obsolete past tense form raught (Old English ræhte). Meaning "arrive at" is early 14c.; that of "succeed in influencing" is from 1660s. Related: Reached; reaching. Reach-me-down "ready-made" (of clothes) is recorded from 1862, from notion of being on the rack in a finished state.
reach (n.): 1520s, from reach (v.); earliest use is of stretches of water.
reacquaint (v.): 1640s, from re- + acquaint. Related: Reacquainted; reacquainting.
react (v.): 1640s, from re- + act (v.). Chemical sense is from 1944. Related: Reacted; reacting. For sense development, see reaction.
reactant: 1901 (n.), 1911 (adj.), from react + -ant.
reaction (n.): 1610s, from re- "again, anew" + action (q.v.). Modeled on French réaction, older Italian reattione, from Medieval Latin reactionem (nominative reactio), from Late Latin react-, past participle stem of reagere "react," from re- "back" + agere "to do, act" (see act).

Originally scientific; physiological sense is attested from 1805; psychological sense first recorded 1887; general sense of "action or feeling in response" (to a statement, event, etc.) is recorded from 1914. Reaction time, "time elapsing between the action of an external stimulus and the giving of a signal in reply," attested by 1874.
reactionary (adj.): 1831, on model of French réactionnaire (19c.), from réaction (see reaction). In Marxist use, opposed to revolutionary and used opprobriously in reference to opponents of communism (1858). As a noun "person considered reactionary in politics," by 1875.
reactivate (v.): 1902, from re- "back, again" + activate. Related: Reactivated; reactivating; reactivation.
reactive (adj.): 1712, from react + -ive. Related: Reactivity.
reactor (n.): "one that reacts," 1835, agent noun in Latin form from react. In nuclear sense, attested from 1945.
read (v.): Old English rædan (West Saxon), redan (Anglian) "to explain, read, rule, advise" (related to ræd, red "advice"), from Proto-Germanic *raedanan (cf. Old Norse raða, Old Frisian reda, Dutch raden, Old High German ratan, German raten "to advise, counsel, guess"), from PIE root *re(i)- "to reason, count" (cf. Sanskrit radh- "to succeed, accomplish," Greek arithmos "number amount," Old Church Slavonic raditi "to take thought, attend to," Old Irish im-radim "to deliberate, consider"). Connected to riddle via notion of "interpret."

Words from this root in most modern Germanic languages still mean "counsel, advise." Transference to "understand the meaning of written symbols" is unique to Old English and (perhaps under English influence) Old Norse raða. Most languages use a word rooted in the idea of "gather up" as their word for "read" (cf. French lire, from Latin legere). Sense of "make out the character of (a person)" is attested from 1610s. The noun meaning "an act of reading" is recorded from 1825. Read up "study" is from 1842; read-only in computer jargon is recorded from 1961.
readable (adj.): 1560s, from read + -able. Related: Readably; readability.
reader (n.): Old English rædere, agent noun from rædan (see read).
readership (n.): 1719, "office of a reader," from reader + -ship. Meaning "total number of readers of a publication" is from 1914.
readily (adv.): early 14c., from ready + -ly (2).
readiness (n.): late 14c., from ready + -ness.
Reading: county town of Berkshire, Old English Readingum (c.900), "(Settlement of) the family or followers of a man called *Read."
readjust (v.): 1742, from re- "back, again" + adjust. Related: Readjusted; readjusting.
readmit (v.): 1610s, from re- "back, again" + admit. Related: Readmitted; readmitting.
ready (adj.): Old English ræde, geræde, from Proto-Germanic *garaidijaz "arranged" (cf. Old Frisian rede, Middle Dutch gereit, Old High German reiti, Middle High German bereite, German bereit, Old Norse greiðr "ready, plain," Gothic garaiþs "ordered, arranged"), from PIE root *reidh-. Lengthened in Middle English by change of ending. Ready-made first attested mid-15c.; ready-to-wear is from 1890.
ready (v.): early 13c., "to administer;" c.1300, "to take aim;" mid-14c., "to prepare, make ready," from ready (adj.). Related: Readied; readying.
reaffirm (v.): 1610s, "to confirm anew," from re- "back, again" + affirm. Meaning "to assert anew" is recorded from 1842. Related: Reaffirmed; reaffirming, reaffirmation.
Reagan: surname, from Irish riagan, literally "little king." Reaganism first recorded 1966, in reference to policies of Ronald W. Reagan (1911-2004), U.S. governor of California 1967-75, U.S. president 1981-89.
Reaganomics (n.): by Feb. 1981, in reference to economic police of U.S. President Ronald Reagan, from Reagan + economics.
reagent (n.): 1797, from re- + agent "substance that produces a chemical reaction."
real (adj.): early 14c., "real, actually existing, true;" mid-15c., "relating to things" (especially property), from Old French reel, from Late Latin realis "actual," from Latin res "matter, thing," of unknown origin. Meaning "genuine" is recorded from 1550s; sense of "unaffected, no-nonsense" is from 1847.
Generally, by the time you are Real, most of your hair has been loved off, and your eyes drop out and you get loose in the joints and very shabby. But these things don't matter at all, because once you are Real you can't be ugly, except to people who don't understand. [Margery Williams, "The Velveteen Rabbit"]
Real estate is first recorded 1660s and retains the oldest English sense of the word. Noun phrase real time is early 19c. as a term in logic and philosophy, 1953 as an adjectival phrase; get real, usually an interjection, was U.S. college slang in 1960s, reached wide popularity c.1987.
real (n.): "small Spanish silver coin," 1580s, from Spanish real, noun use of real (adj.) "regal," from Latin regalis "regal" (see regal). Especially in reference to the real de plata, which circulated in the U.S. till c.1850 and in Mexico until 1897.
realia (n.): 1952, neuter plural of Late Latin realis "actual, real" (see real (adj.)).
realign (v.): 1876 in reference to railroad tracks; 1923, in reference to European international relations, from re- "back, again" + align. Related: Realigned; realigning.
realignment (n.): 1850, from realign + -ment.
realise (v.): chiefly British English spelling of realize; for suffix, see -ize. Related: Realisation; realised; realising.
realism (n.): 1817, from real (adj.) + -ism; after French réalisme or German Realismus; from Late Latin realis "real." Opposed to idealism in philosophy, art, etc. In reference to scholastic doctrine of Thomas Aquinas (opposed to nominalism) it is recorded from 1826. Meaning "close resemblance to the scene" (in art, literature, etc., often with reference to unpleasant details) is attested from 1856.
realist (n.): 1690s, in philosophy; see realism.
realistic (adj.): "true to reality" (in art, etc.), 1854, from real (adj.) + -istic. Meaning "having a practical view of life" (opposed to idealistic) is attested from 1831. Related: Realistically.
reality (n.): 1540s, originally a legal term in the sense of "fixed property," from Medieval Latin realitatem (nominative realitas), from Late Latin realis; meaning "real existence" is from 1640s.
realization (n.): 1610s, "action of making real," from realize + -ation. Meaning "action of forming a clear concept" is from 1828. Related: Realizational.
realize (v.): 1610s, "bring into existence," from French réaliser "make real," from Middle French real "actual" (see real (adj.)). Sense of "understand clearly" is first recorded 1775. Related: Realized; realizing.
reallocate (v.): by 1868, from re- + allocate. Related: Reallocated; reallocating; reallocation.
really (adv.): c.1400, originally in reference to the presence of Christ in the Eucharist, from real (adj.) + -ly (2). Sense of "actually" is from early 15c. Purely emphatic use dates from c.1600; interrogative use (oh, really?) is first recorded 1815.
realm (n.): late 13c., from Old French reaume, probably from roiaume "kingdom," altered (by influence of Latin regalis "regal") from Gallo-Romance *regiminem, accusative form of Latin regimen "system of government, rule" (see regimen).
realpolitik (n.): 1914, from German realpolitk, literally "practical politics."