Online Etymology Dictionary

infernal (adj.): late 14c., in reference to the underworld, from Old French enfernal, infernal (12c.), from Late Latin infernalis "of the lower regions," from infernus "hell" (Ambrose), literally "the lower (world)," noun use of Latin infernus "lower, lying beneath," from infra "below" (see infra-). Meaning "devilish, hateful" is from early 15c. For the name of the place, or things which resemble it, the Italian form inferno has been used in English since 1834, from Dante. Related: Infernally.
inferno (n.): 1834, from Italian inferno, from Latin infernus (see infernal).
infertile (adj.): 1590s, from French infertile (late 15c.), from Late Latin infertilis "unfruitful," from in- "not, opposite of" (see in- (1)) + fertilis (see fertile).
infertility (n.): c.1600, from Late Latin infertilitatem (nominative infertilitas), from infertilis (see infertile).
infest (v.): late 15c., "to attack, assail, hurt, distress, annoy," from Middle French infester, from Latin infestare "to attack, disturb, trouble," from infestus "hostile, dangerous," originally "inexorable, not able to be handled," from in- "not, opposite of" (see in- (1)) + -festus "(able to be) seized." Sense of "swarm over in large numbers" first recorded c.1600. Related: Infested; infesting.
infestation (n.): early 15c., from Late Latin infestationem (nominative infestatio) "a troubling, disturbing, molesting," noun of action from past participle stem of infestare (see infest).
infibulate (v.): 1620s, from Latin infibulatus, past participle of infibulare "to close with a clasp," from in- "not, opposite of" (see in- (1)) + fibula "a clasp, pin" (see fibula). Related: Infibulated.
infibulation (n.): 1640s, noun of action from infibulate.
infidel: mid-15c. (adjective and noun), from Middle French infidèle, from Latin infidelis "unfaithful, not to be trusted," later "unbelieving," from in- "not, opposite of" (see in- (1)) + fidelis "faithful" (see fidelity). In 15c. "a non-Christian" (especially a Saracen); later "one who does not believe in religion" (1520s). Also used to translate Arabic qafir, which is from a root meaning "to disbelieve, to deny," strictly referring to all non-Muslims but virtually synonymous with "Christian;" hence, from a Muslim or Jewish point of view, "a Christian" (1530s; see kaffir).
infidelity (n.): c.1400, "want of faith, unbelief in religion; false belief, paganism;" also (early 15c.) "unfaithfulness or disloyalty to a person" (originally to a sovereign, by 16c. to a lover or spouse), from French infidélité, from Latin infidelitatem (nominative infidelitas) "unfaithfulness, faithlessness," noun of quality from infidelis (see infidel).
infield (n.): 1733, "the land of a farm which lies nearest the homestead," from in + field. Baseball diamond sense first attested 1867. Related: Infielder.
infiltrate (v.): 1758, of fluids, from in- (2) "in" + filtrate. Related: Infiltrated; infiltrating. Military sense of "penetrate enemy lines" attested from 1934.
infiltration (n.): early 15c., "a knitting together," noun of action from infiltrate. In physics, from 1796. Figurative sense of "a passing into" (anything immaterial) is from 1840; military sense of "stealthy penetration of enemy lines" dates from 1930.
infinite (adj.): late 14c., "eternal, limitless," also "extremely great in number," from Old French infinit "endless, boundless," and directly from Latin infinitus "unbounded, unlimited," from in- "not, opposite of" (see in- (1)) + finitus "defining, definite," from finis "end" (see finish). The noun meaning "that which is infinite" is from 1580s.
infinitely (adv.): early 15c., from infinite + -ly (2).
infinitesimal (adj.): 1710 (1650s as a noun), "infinitely small," from Modern Latin infinitesimus, from Latin infinitus "infinite" (see infinite) + -esimus, as in centesimus "hundredth." Related: Infinitesimally.
infinitive (n.): "simple, uninflected form of a verb," 1510s (mid-15c. as an adjective), from Late Latin infinitivus "unlimited, indefinite," from Latin infinitus (see infinite). "Indefinite" because not having definite person or number.
infinitude (n.): 1640s, from Medieval Latin *infinitudo, from Latin infinitus on model of multitudo, magnitudo; see infinite. Cf. French infinitude (1610s).
infinity (n.): late 14c., from Old French infinité "infinity; large number or quantity" (13c.), from Latin infinitatem (nominative infinitas) "boundlessness, endlessness," from infinitus boundless, unlimited" (see infinite). Infinitas was used as a loan-translation of Greek apeiria "infinity," from apeiros "endless."
infirm (adj.): late 14c., "weak, unsound" (of things), from Latin infirmus "weak, frail, feeble" (figuratively "superstitious, pusillanimous, inconstant"), from in- "not, opposite of" (see in- (1)) + firmus (see firm (adj.)). Of persons, "not strong, unhealthy," first recorded c.1600. As a noun from 1711.
infirmary (n.): mid-15c., "sick bay in a monastery," from Medieval Latin infirmaria "a place for the infirm," from Latin infirmus "weak, frail," (see infirm). The common name for a public hospital in 18c. England.
infirmity (n.): late 14c., "disease, sickness; lack of capability, weakness," from Latin infirmitatem (nominative infirmitas) "want of strength, weakness, feebleness," noun of quality from infirmus (see infirm). Cf. Middle French infirmité, Old French enfermete.
inflame (v.): mid-14c., "to set on fire with passion," from Latin inflammare "to set on fire, kindle," figuratively "to rouse, excite," from in- "in" (see in- (2)) + flammare "to flame," from flamma "flame" (see flame (n.)). Literal sense of "to cause to burn" first recorded in English late 14c.
inflammable (adj.): early 15c., in medicine, "liable to inflammation," from Middle French inflammable and directly from Medieval Latin inflammabilis, from Latin inflammare (see inflame). As "able to be set alight," c.1600. Related: Inflammability.
inflammation (n.): "redness or swelling in a body part," early 15c., from Middle French inflammation and directly from Latin inflammationem (nominative inflammatio) "a setting on fire," noun of action from past participle stem of inflammare (see inflame). Literal sense in English from 1560s.
inflammatory (adj.): 1680s (n.), from Latin inflammat-, past participle stem of inflammare (see inflame) + -ory. As an adjective, "tending to rouse passions or desires," 1711; from 1732 in pathology.
inflate (v.): early 15c., "cause to swell," from Latin inflatus, past participle of inflare "to blow into, inflate" (see inflation). Economics sense from 1844. In some senses a back-formation from inflation. Related: Inflatable; inflated; inflating.
inflation (n.): mid-14c., "swelling caused by gathering of 'wind' in the body; flatulence;" figuratively, "outbursts of pride," from Latin inflationem (nominative inflatio) "a puffing up; flatulence," noun of action from past participle stem of inflare "blow into, puff up," from in- "into" (see in- (2)) + flare "to blow" (see blow (v.1)). Monetary sense of "enlargement of prices" (originally by an increase in the amount of money in circulation) first recorded 1838 in American English.
inflationary (adj.): 1916, from inflation + -ary.
inflect (v.): early 15c., "to bend inward," from Latin inflectere (past participle inflexus) "to bend in, bow, curve," figuratively, "to change," from in- "in" (see in- (1)) + flectere "to bend" (see flexible). Grammatical sense is attested 1660s; pronunciation sense (in inflection) is c.1600. Related: Inflected; inflecting.
inflection (n.): early 15c., from Middle French inflexion and directly from Latin inflexionem (nominative inflexio) "a bending, inflection, modification," noun of action from past participle stem of inflectere (see inflect). For spelling, see connection. Grammatical sense is from 1660s.
inflexibility (n.): 1610s, from inflexible + -ity.
inflexible (adj.): late 14c., "incapable of being bent, physically rigid," also figuratively, "unbending in temper or purpose," from Middle French inflexible and directly from Latin inflexibilis, from inflexus, past participle of inflectere (see inflect). In early 15c. an identical word had an opposite sense, "capable of being swayed or moved," from in- "in, on." Related: Inflexibly.
inflexion (n.): see inflection.
inflict (v.): 1560s, from Latin inflictus, past participle of infligere "to strike or dash against," from in- "on, against" (see in- (2)) + fligere (past participle flictus) "to dash, strike" (see afflict). You inflict trouble on someone; you afflict someone with trouble. Shame on you.
infliction (n.): 1530s, from Late Latin inflictionem (nominative inflictio) "an inflicting, a striking against," noun of action from past participle stem of Latin infligere (see inflict).
inflight (adj.): also in-flight, "during a flight," 1945, from in + flight.
inflorescence (n.): 1760, from Modern Latin inflorescentia, from Late Latin inflorescentem (nominative inflorescens) "flowering," present participle of Latin inflorescere "to come to flower," from in- "in" (see in- (2)) + florescere "to begin to bloom" (see flourish).
inflow (n.): 1839, from in + flow (n.).
influence (n.): late 14c., an astrological term, "streaming ethereal power from the stars acting upon character or destiny of men," from Old French influence "emanation from the stars that acts upon one's character and destiny" (13c.), also "a flow of water," from Medieval Latin influentia "a flowing in" (also used in the astrological sense), from Latin influentem (nominative influens), present participle of influere "to flow into," from in- "into, in, on, upon" (see in- (2)) + fluere "to flow" (see fluent). Meaning "exercise of personal power by human beings" is from mid-15c.; meaning "exertion of unseen influence by persons" is from 1580s (a sense already in Medieval Latin, e.g. Aquinas). Under the influence "drunk" first attested 1866.
influence (v.): 1650s, from influence (n.). Related: Influenced; influencing.
influent (adj.): mid-15c., "abundant," in reference to occult power of the stars, etc., also of grace, from Latin influentem (nominative influens), present participle of influere “to flow in” (see influence).
influential (adj.): "powerful," 1650s, from Medieval Latin influentialis, from influentia (see influence). Earlier in an astrological sense (1560s).
influenza (n.): 1743, borrowed during an outbreak of the disease in Europe, from Italian influenza "influenza, epidemic," originally "visitation, influence (of the stars)," from Medieval Latin influentia (see influence). Used in Italian for diseases since at least 1504 (cf. influenza di febbre scarlattina "scarlet fever") on notion of astral or occult influence. The 1743 outbreak began in Italy. Often applied since mid-19c. to severe colds.
influx (n.): 1620s, from French influx (1540s) or directly from Late Latin influxus "a flowing in," from past participle stem of influere "to flow in" (see influence). Originally of rivers, air, light, spiritual light, etc.; used of people from 1650s.
info (n.): 1906, short for information.
info-: prefix abstracted from information.
infomercial (n.): 1983, from info- + commercial. Before infomercial was the print form, advertorial (1961).
inform (v.): early 14c., "to train or instruct in some specific subject," from Old French informer "instruct, inform, teach," and directly from Latin informare "to shape, form," figuratively "train, instruct, educate," from in- "into" (see in- (2)) + formare "to form, shape," from forma "form" (see form (n.)). Varied with enform until c.1600. Sense of "report facts or news" first recorded late 14c. Related: Informed; informing.
informal (adj.): mid-15c., "lacking form; not in accordance with the rules of formal logic," from in- (1) "not, opposite of" + formal. Meaning "irregular, unofficial" is from c.1600. Sense of "done without ceremony" is from 1828. Related: Informally.