Online Etymology Dictionary

registrar (n.): 1670s, shortening of registrary (1540s), from Medieval Latin registrarius "one who keeps a record" (related to register).
registration (n.): 1560s, from Medieval Latin registrationem, noun of action from past participle stem of registrare (see register (v.)).
registry (n.): 1580s, "act of registering;" see register + -y (1). Meaning "book of record" is from 1620s.
regnant (adj.): c.1600, from Latin regnant-, present participle stem of regnare "to reign" (see reign).
regolith (n.): 1897, from Greek rhegos "rug, blanket" + lithos "stone" (see litho-).
regress (n.): late 14c. (n.), "act of going back," from Latin regressus "a return," from regress-, past participle stem of regredi "to go back," from re- "back" (see re-) + gradi "to step, walk" (see grade (n.)).
regress (v.): "to move backward," 1823, from regress (n.). The psychological sense of "to return to an earlier stage of life" is attested from 1926. Related: Regressed; regressing.
regression (n.): 1510s, from Latin regressionem, noun of action from regredi (see regress (n.)).
regressive (adj.): 1630s; see regress + -ive. In reference to taxation, it is attested from 1889. Related: Regressively.
regret (v.): "to remember with distress or longing," c.1300, from Old French regreter "long after, bewail, lament someone's death," from re-, intensive prefix + -greter, possibly from Frankish (cf. Old English grætan "to weep;" Old Norse grata "to weep, groan"), from Proto-Germanic *gretan "weep." Replaced Old English ofþyncan, from of- "off, away," here denoting opposition + þyncan "seem, seem fit" (as in methinks).
regret (n.): 1530s, from regret (v.).
regretful (adj.): 1640s, "full of regret," from regret + -ful. Regretfully incorrectly used in place of regrettably since at least 1965. "A regrettable use, prob. after HOPEFULLY" [OED].
regrettable (adj.): c.1600, "deserving of regret," from regret + -able. Related: Regrettably.
regroup (v.): also re-group, 1885, from re- "again" + group (v.). Related: Regrouped; regrouping.
regular (adj.): late 14c., from Old French reguler, from Late Latin regularis "continuing rules for guidance," from Latin regula "rule," from PIE *reg- "move in a straight line" (see regent). Earliest sense was of religious orders (the opposite of secular). Extended 16c. to shapes, etc., that followed predictable or uniform patterns; sense of "normal" is from 1630s; meaning "real, genuine" is from 1821. Noun meaning "a regular customer" is recorded from 1852.
regularly (adv.): 1520s, from regular + -ly (2).
regulate (v.): 1630s, from Late Latin regulatus, past participle of regulare "to control by rule, direct" (5c.), from Latin regula "rule" (see regular). Related: Regulated; regulating.
regulation (n.): 1670s, "act of regulating," noun of action from regulate. Meaning "rule for management" is from 1715. Related: Regulations.
regulator (n.): 1650s, agent noun in Latin form from regulate. In English history, from 1680s; in American history, from 1767, applied to local posses that kept order (or disturbed it) in rural regions. As a clock or watch used to set the time of other pieces, from 1758.
Regulus (n.): bright star in constellation Leo, 1550s, from Latin, literally "little king," diminutive of rex "king;" probably a translation of Basiliskos "little king," a Hellenistic Greek name for the star, mentioned in Geminos and Ptolemy (in the "Almagest," though elsewhere in his writings it is usually "the star on the heart of Leo"); perhaps a translation of Lugal "king," the star's Babylonian name.
regurgitate (v.): 1650s (intransitive), 1753 (transitive), from past participle stem of Medieval Latin regurgitare (see regurgitation). Related: Regurgitated; regurgitating.
regurgitation (n.): c.1600, from Medieval Latin regurgitationem, noun of action from regurgitare "to overflow," from Late Latin re- "back" + gurgitare "engulf, flood" (found in Latin ingurgitare "to pour in"), from gurges "whirlpool, gorge, abyss." Meaning "to vomit" first attested 1753.
rehab: 1948 as a shortening of rehabilitation (originally of service members returning from World War II). As a verb in reference to houses, by 1975. Related: Rehabbed; rehabbing.
rehabilitate (v.): 1570s, from past participle stem of Medieval Latin rehabilitare (see rehabilitation). Related: Rehabilitated; rehabilitating.
rehabilitation (n.): 1530s, from Middle French réhabilitation, from Medieval Latin rehabilitationem "restoration," from rehabilitatus, past participle of rehabilitare, from re- "again" + habitare "make fit," from Latin habilis "easily managed, fit" (see able). Specifically of criminals, addicts, etc., from 1940.
rehash (v.): 1822, from re- "again" + hash (v.). Related: Rehashed; rehashing. The noun is first attested 1849.
rehearsal (n.): late 14c., "restatement," from rehearse + -al (2). Sense in theater and music is from 1570s. Rehearsal dinner attested by 1953.
rehearse (v.): c.1300, "to give an account of," from Anglo-French rehearser, Old French rehercier "to go over again, repeat," literally "to rake over," from re- "again" (see re-) + hercier "to rake, harrow" (see hearse). Meaning "to say over again" is from mid-14c.; sense of "practice a play, part, etc." is from 1570s. Related: Rehearsed; rehearsing.
reheat (v.): 1727, from re- "again" + heat (v.). Related: Reheated; reheating.
Rehoboth: Hebrew Rehobhoth, literally "wide places" (Gen. xxvi:22).
reich (n.): German, "kingdom, realm, state" (see Reichstag). Used from 1871-1945 to refer to the German state, Germany. Most notoriously in Third Reich (see third); there never was a First or Second in English usage.
Reichstag (n.): "German imperial parliament" (1871-1918), earlier used of the parliament of the North German Confederacy, 1867, from German Reichstag, from Reich "empire, kingdom" (from Old High German rihhi, related to Old English rice, from Proto-Germanic *rikja "rule;" see rich) + Tag "assembly," literally "day" (see day).
reification (n.): 1846, "act of materializing," from Latin re-, stem of res "thing" + -fication. In Marxist jargon, translating German Verdinglichung.
reify (v.): 1854, from re-, stem of Latin res "thing" + -fy. Related: Reified; reifying.
reign (n.): early 13c., "kingdom," from Old French reigne, from Latin regnum "kingship, dominion, rule, realm," related to regere (see regal). Meaning "period of rule" first recorded mid-14c.
reign (v.): "to hold or exercise sovereign power," late 13c., from Old French regner, from Latin regnare, from regnum (see reign (n.)). Related: Reigned; reigning.
reignite (v.): 1863, from re- + ignite. Related: Reignited; reigniting.
reimburse (v.): 1610s, from re- "back" + imburse "to pay, enrich," literally "put in a purse" (c.1530), from Middle French embourser, from Old French em- "in" + borser "to get money," from borse "purse," from Medieval Latin bursa (see purse). Related: Reimbursed; reimbursing.
rein (n.): c.1300, "strap fastened to a bridle," from Old French rene, probably from Vulgar Latin *retina "a bond, check," back-formation from Latin retinere "hold back" (see retain). The verb is c.1300, from the noun. Figurative extension "put a check on" first recorded 1580s. Related: Reined; reining.
reincarnate (v.): 1858, from re- + incarnate. Related: Reincarnated; reincarnating. As an adjective from 1882.
reincarnation (n.): 1858, from re- "back, again" + incarnation.
reindeer (n.): c.1400, from Old Norse hreindyri "reindeer," from dyr "animal" (see deer) + hreinn, the usual name for the animal, from Proto-Germanic *khrainaz (cf. Old English hran "reindeer," German Renn). Probably cognate with Greek krios "ram," but folk etymology associates it with rennen "to run."
reinforce (v.): c.1600, originally in military sense, from re- "again" + enforce (cf. re-enforce). Related: Reinforced; reinforcing.
reinforcement (n.): 1610s, from reinforce + -ment. Related: Reinforcements.
reinstate (v.): 1590s, from re- + instate. Related: Reinstated; reinstating.
reinstatement (n.): 1797, from reinstate + -ment.
reintegrate (v.): 1580s, from re- + integrate. Related: Reintegrated; reintegrating.
reintegration (n.): c.1600, from French réintegration (15c.) or directly from Medieval Latin reintegrationem; see reintegrate + -ion.
reintroduction (n.): 1660s, from re- + introduction.
reinvent (v.): 1680s, from re- + invent. Related: Reinvented; reinventing. Phrase reinvent the wheel "do redundant work" attested by 1971.