Online Etymology Dictionary

relational (adj.): 1660s, from relation + -al (1).
relationship (n.): 1640s, "sense or state of being related," from relation + -ship. Specifically of romantic or sexual relationships by 1944.
relative (n.): late 14c., "a relative pronoun," from Old French relatif (13c.), from Late Latin relativus "having reference or relation," from Latin relatus, past participle of referre "to refer" (see refer). Meaning "person in the same family" first recorded 1650s.
relative (adj.): early 15c., "having reference," from Middle French relatif and directly from Late Latin relativus (see relative (n.)). Meaning "compared to each other" is from 1590s; that of "depending on a relationship to something else" is from 1610s.
relatively (adv.): "in relation to something else," 1560s, from relative (adj.) + -ly (2).
relativism (n.): 1865, in philosophy, from relative (adj.) + -ism. Compare relativist.
relativist (n.): 1857, from relative + -ist. As an adjective from 1914. Related: Relativistic.
relativity (n.): 1834, "fact or condition of being relative" (apparently coined by Coleridge, of God, in "Notes on Waterland's Vindication of Christ's Divinity"), from relative (adj.) + -ity. In scientific use, connected to the theory of Albert Einstein (1879-1955), published 1905 (special theory of relativity) and 1915 (general theory of relativity), but the word was used in roughly this sense by J.C. Maxwell in 1876.
relator (n.): "informer," c. 1600, from Latin relator, agent noun; see relate.
relaunch (v.): 1745, from re- + launch (v.). Related: Relaunched; relaunching. As a noun from 1970.
relax (v.): late 14c., "to make (something) less compact or dense," from Old French relaschier "set free; soften; reduce" (14c.), from Latin relaxare "relax, loosen, open, stretch out, widen again; make loose," from re- "back" (see re-) + laxare "loosen," from laxus "loose" (see lax). Of persons, "to become less formal," from 1837. Meaning "decrease tension" is from early 15c.; intransitive sense of "to become less tense" is recorded from 1935. Related: Relaxed; relaxing.
relaxant (adj.): 1771, from Latin relaxantem (nominative relaxans), present participle of relaxare (see relax). As a noun from 1832.
relaxation (n.): late 14c., "rupture; mid-15c., "remission of a burden or penalty," from Old French relaxacion (14c.) and directly from Latin relaxationem (nominative relaxatio) "an easing, mitigation, relaxation," noun of action from past participle stem of relaxare (see relax). Meaning "relief from hard work or ordinary cares" is from 1540s.
relay (n.): late 14c., "hounds placed along a line of chase," from Middle French relai "reserve pack of hounds or other animals" (13c.), from Old French relaier "to exchange tired animals for fresh," literally "leave behind," from re- "back" (see re-) + laier "to leave" (see delay (v.)). The etymological sense is "to leave (dogs) behind (in order to take fresh ones)." Of horses, 1650s. Electromagnetic sense first recorded 1860. As a type of foot-race, it is attested from 1898.
relay (v.): c. 1400, "to set a pack of (fresh) hounds after a quarry;" also "change horses," from Old French relaiier, from relai (see relay (n.)). Related: Relayed; relaying.
release (v.): c. 1300, "to withdraw, revoke (a decree, etc.), cancel, lift; remit," from Old French relaissier, relesser "to relinquish, quit, let go, leave behind, abandon, acquit," variant of relacher "release, relax," from Latin relaxare "loosen, stretch out" (see relax), source also of Spanish relajar, Italian relassare.

Meaning "alleviate, ease" is mid-14c., as is sense of "free from (duty, etc.); exonerate." From late 14c. as "grant remission, forgive; set free from imprisonment, military service, etc." Also "give up, relinquish, surrender." In law, c. 1400, "to grant a release of property." Of press reports, attested from 1904; of motion pictures, from 1912; of music recordings, from 1962. As a euphemism for "to dismiss, fire from a job" it is attested in American English since 1904. Related: Released; releasing.
release (n.): early 14c., "abatement of distress; means of deliverance," from Old French relais, reles (12c.), a back-formation from relesser, relaissier (see release (v.)). In law, mid-14c., "transferring of property or a right to another;" late 14c. as "release from an obligation; remission of a duty, tribute, etc." Meaning "act and manner of releasing" (a bow, etc.) is from 1871. Sense of "action of publication" is from 1907.
relegate (v.): 1590s "to banish, send into exile," from Latin relegatus, past participle of relegare "remove, dismiss, banish, send away, schedule, put aside," from re- "back" (see re-) + legare "send with a commission" (see legate). Meaning "place in a position of inferiority" is recorded from 1790. Related: Relegated; relegating; relegable.
relegation (n.): 1580s, from Latin relegationem (nominative relegatio), noun of action from past participle stem of relegare (see relegate).
relent (v.): late 14c., "to melt, soften, dissolve," from re- + Latin lentus "slow, viscous, supple" (see lithe), perhaps on model of Old French rallentir. Sense of "become less harsh or cruel" first recorded 1520s. The notion probably is of a hard heart melting with pity. Related: Relented; relenting.
relentless (adj.): 1590s, from relent + -less. Related: Relentlessly; relentlessness.
relevance (n.): 1733; see relevant + -ance. Related: Relevancy (1560s).
relevant (adj.): "pertinent to the matter at hand," 1550s, from Middle French relevant "depending upon," originally "helpful," from Medieval Latin relevantem (nominative relevans), from stem of Latin relevare "to lessen, lighten" (see relieve). Not generally used until after 1800.
releve (n.): 1825 as a dish; 1930 in ballet, from French relevé, 19th century verbal noun from past participle of relever (see relieve), so, literally "raised up."
reliability (n.): 1816, from reliable + -ity.
reliable (adj.): 1560s, raliabill, Scottish; see rely + -able. Not common before 1850; and sometimes execrated thereafter in Britain as an Americanism because it involves a use of -able different from its use in provable, etc., but defended (by OED, Century Dictionary, etc.) on grounds of use of the suffix in available, laughable, etc.. Related: Reliably.
reliance (n.): c. 1600, from rely + -ance.
reliant (adj.): 1856; see rely + -ant. Because it means "dependent (on)" it would seem an odd name for an automobile, but Chrysler (Plymouth) nonetheless chose it as one in 1981.
relic (n.): early 13c., "body part or other object from a holy person," from Old French relique (11c., plural reliques), from Late Latin reliquiæ (plural) "remains of a martyr," in classical Latin "remains, remnants," noun use of fem. plural of reliquus "remaining, that which remains," related to relinquere (perfective reliqui) "to leave behind" (see relinquish). Sense of "remains, ruins" is from early 14c. Old English used reliquias, directly from Latin.
relict (n.): "a widow," mid-15c., from Old French relict, fem. relicte "person or thing left behind" (especially a widow) and directly from Medieval Latin relicta "a widow," noun use of fem. of relictus "abandoned, left behind," past participle adjective from Latin relinquere "to leave behind" (see relinquish).
reliction (n.): in law, "a recession of the sea from the land," 1670s, from Latin relictionem (nominative relictio), noun of action from past participle stem of relinquere (see relinquish).
relief (n.1): late 14c., "alleviation of distress, hunger, sickness, etc; state of being relieved; that which mitigates or removes" (pain, grief, evil, etc.)," from Anglo-French relif, from Old French relief "assistance," literally "a raising, that which is lifted," from stressed stem of relever (see relieve). Meaning "aid to impoverished persons" is attested from c. 1400; that of "deliverance of a besieged town" is from c. 1400. Earlier in English as "that which is left over or left behind," also "feudal payment to an overlord made by an heir upon taking possession of an estate" (both c. 1200).
relief (n.2): "projection of figure or design from a flat surface," c. 1600, from French relief, from Italian rilievo, from rilevare "to raise," from Latin relevare "to raise, lighten" (see relieve).
relieve (v.): late 14c., "alleviate (pain, etc.), mitigate; afford comfort; allow respite; diminish the pressure of," also "give alms to, provide for;" also figuratively, "take heart, cheer up;" from Old French relever "to raise, relieve" (11c.) and directly from Latin relevare "to raise, alleviate, lift up, free from a burden," from re-, intensive prefix (see re-), + levare "to lift up, lighten," from levis "not heavy" (see lever).

The notion is "to raise (someone) out of trouble." From c. 1400 as "advance to the rescue in battle;" also "return from battle; recall (troops)." Meaning "release from duty" is from early 15c. Related: relieved; relieving.
reliever (n.): late 15c., agent noun from relieve. Baseball sense ("relief pitcher") is attested by 1945.
religate (v.): "bind together," 1590s from Latin religatus, past participle of religare "fasten, bind fast" (see rely). Related: Religated; religating.
religieuse (n.): "a nun, a religious woman," 1690s, from French, fem. of religieux "monk," noun use of adjective meaning "religious" (see religious). As a type of pastry, attested from 1929.
religion (n.): c. 1200, "state of life bound by monastic vows," also "conduct indicating a belief in a divine power," from Anglo-French religiun (11c.), Old French religion "piety, devotion; religious community," and directly from Latin religionem (nominative religio) "respect for what is sacred, reverence for the gods; conscientiousness, sense of right, moral obligation; fear of the gods; divine service, religious observance; a religion, a faith, a mode of worship, cult; sanctity, holiness," in Late Latin "monastic life" (5c.).

According to Cicero derived from relegere "go through again" (in reading or in thought), from re- "again" (see re-) + legere "read" (see lecture (n.)). However, popular etymology among the later ancients (Servius, Lactantius, Augustine) and the interpretation of many modern writers connects it with religare "to bind fast" (see rely), via notion of "place an obligation on," or "bond between humans and gods." In that case, the re- would be intensive. Another possible origin is religiens "careful," opposite of negligens. In English, meaning "particular system of faith" is recorded from c. 1300; sense of "recognition of and allegiance in manner of life (perceived as justly due) to a higher, unseen power or powers" is from 1530s.

To hold, therefore, that there is no difference in matters of religion between forms that are unlike each other, and even contrary to each other, most clearly leads in the end to the rejection of all religion in both theory and practice. And this is the same thing as atheism, however it may differ from it in name. [Pope Leo XIII, Immortale Dei, 1885]
religiosity (n.): late 14c., from Old French religiosete and directly from Late Latin religiositas "religiousness," from religiosus (see religious).
religious (adj.): c. 1200, "devout, pious, imbued with or expressive of religious devotion," from Anglo-French religius, Old French religious (12c., Modern French religieux) and directly from Latin religiosus, from religio (see religion). Meaning "pertaining to religion" is from 1530s. Transferred sense of "scrupulous, exact" is recorded from 1590s. Related: Religiousness.
religiously (adv.): late 14c., "in a religous manner," from religious + -ly (2). Transferred sense of "strictly, scrupulously" attested by 1570s.
relinquish (v.): mid-15c., "desert, abandon;" late 15c., "give up, desist," from Old French relinquiss-, present participle stem of relinquir (12c.), from Latin relinquere "leave behind, forsake, abandon, give up," from re- "back" (see re-) + linquere "to leave," from PIE *linkw-, from root *leikw- "to leave behind" (source also of Sanskrit reknas "inheritance, wealth," rinakti "leaves;" Greek leipein "to leave, be lacking;" Gothic leihvan, Old English lænan "to lend;" Old High German lihan "to borrow;" Old Norse lan "loan"). Related: Relinquished; relinquishing.
reliquary (n.): "receptacle for keeping relics," 1650s, from French reliquaire (14c.), from relique (see relic).
relique (n.): Frenchified spelling of relic (q.v.).
reliquiae (n.): "remains," 1650s, Latin plural of reliquus "remainder, residue," noun use of adjective meaning "that is left, remaining, left over," related to relinquere (perfective reliqui) "to leave behind" (see relinquish).
relish (n.): 1520s, "taste, flavor," alteration of reles "scent, taste, aftertaste," (c. 1300), from Old French relais, reles, "something remaining, that which is left behind," from relaisser "to leave behind" (see release (v.)). Meaning "enjoyment of the taste or flavor of something" is attested from 1640s. Sense of "condiment, that which imparts flavor" is first recorded 1797. The stuff you put on hot dogs is a sweet green pickle relish.
relish (v.): 1560s "give flavor to" (implied in relished), from relish (n.). The transferred sense of "to enjoy, take pleasure in" is from 1590s. Related: Relishing.
relive (v.): 1540s, "to come to life again" (also "to restore to life again"), from re- "back, again" + live (v.). Meaning "to experience over again" is attested from c. 1711. Related: Relived; reliving.
reload (v.): 1778, from re- "back, again" + load (v.). Of a firearm from 1853. Related: Reloaded; reloading.
relocate (v.): 1822, transitive, "to move (something, originally a road) to another place," from re- "back, again" + locate (v.). Intransitive sense of "settle again" is from 1841. Related: Relocated; relocating.