Online Etymology Dictionary

infelicity (n.): late 14c., "unhappiness," from Latin infelicitas "bad luck, misfortune, unhappiness," from infelix (genitive infelicis) "unfruitful, barren; unfortunate, unhappy; causing misfortune, unlucky," from in- "not, opposite of" (see in- (1)) + felix (see felicity). Meaning "inappropriateness, unhappiness as to occasion" is from 1610s.
infer (v.): in logic, "to 'bring in' as a conclusion of a process of reasoning," 1520s, from Latin inferre "bring into, carry in; deduce, infer, conclude, draw an inference; bring against," from in- "in" (see in- (2)) + ferre "carry, bear," from PIE *bher- (1) "to bear, to carry, to take" (source also of Sanskrit bharati "carries;" Avestan baraiti "carries;" Old Persian barantiy "they carry;" Armenian berem "I carry;" Greek pherein "to carry;" Old Irish beru/berim "I catch, I bring forth;" Gothic bairan "to carry;" Old English and Old High German beran, Old Norse bera "barrow;" Old Church Slavonic birati "to take;" Russian brat' "to take," bremya "a burden").

General sense of "draw a conclusion" is first attested 1520s; intransitive sense is from 1570s.
inference (n.): 1590s, "action of inferring;" 1610s, "that which is inferred;" from Medieval Latin inferentia, from Latin inferentem (nominative inferens), present participle of inferre "bring into; conclude, deduce" (see infer).
inferential (adj.): 1650s, from Medieval Latin inferentia (see inference) + -al (1). Related: Inferentially.
inferior (adj.): early 15c., of land, "low, lower down, lower in position," from Latin inferior "lower, further down" (also used figuratively), comparative of inferus (adj.) "that is below or beneath," from infra "below" (see infra-). Meaning "lower in degree, rank, grade, or importance" is from 1530s; absolutely, "of low quality or rank," also from 1530s.
inferior (n.): "person inferior to another in rank, etc.," early 15c., from inferior (adj.).
inferiority (n.): "state of being inferior," 1590s, probably from Medieval Latin *inferioritas; see inferior + -ity. Inferiority complex first attested 1919.
The surrender of life is nothing to sinking down into acknowledgment of inferiority. [John C. Calhoun]
infernal (adj.): late 14c., "of or pertaining to the underworld," (ancient Tartarus, the sunless abode of the dead, or the Christian Hell), from Old French enfernal, infernal "of Hell, hellish" (12c.), from Late Latin infernalis "of or belonging to the lower regions," from infernus "hell" (Ambrose), in classical Latin "the lower (world)," noun use of infernus "lower, lying beneath, underground, of the lower regions," from infra "below" (see infra-).

Pluto was infernus rex, and Latin inferi meant "the inhabitants of the infernal regions, the dead." Association of the word with fire and heat is via the Christian conception of Hell. Meaning "devilish, hateful" is from early 15c.; meaning "suitable for or appropriate to Hell" is from c. 1600. As a name of Hell, or a word for things which resemble it, the Italian form inferno has been used in English since 1834, via Dante. Related: Infernally.
inferno (n.): 1834, "Hell, the infernal regions," from Italian inferno, from Late Latin infernus "Hell," in classical Latin "the lower world" (see infernal). As "a large, raging fire" from 1928.
infertile (adj.): 1590s, from French infertile (15c.), from Late Latin infertilis "unfruitful," from in- "not, opposite of" (see in- (1)) + Latin fertilis "fruitful, productive" (see fertile).
infertility (n.): c. 1600, from French infertilité (16c.) or directly from Late Latin infertilitatem (nominative infertilitas), from infertilis "not fertile, unfruitful" (see infertile).
infest (v.): late 15c., "to attack, assail, hurt, distress, annoy," from Old French infester (14c.), from Latin infestare "to attack, disturb, trouble," from infestus "unsafe, hostile, threatening, 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, attack parasitically" first recorded c. 1600. Related: Infested; infesting.
infestation (n.): early 15c., "a being infested," from Old French infestacion, from Late Latin infestationem (nominative infestatio) "a troubling, a disturbing, a molesting," noun of action from past participle stem of Latin infestare "to attack, disturb" (see infest).
infibulate (v.): "to clasp, confine with a buckle, ring, clasp, or the like," especially of the sexual organs, to prevent copulation, 1620s, from Latin infibulatus, past participle of infibulare "to close with a clasp," from in- "on" (see in- (2)) + fibula "a clasp, pin" (see fibula). Related: Infibulated.
This operation was very generally practised in antiquity upon both young men and young women, but in later times chiefly upon the latter; and it is said to be still in use in some parts of the East. [Century Dictionary, 1902]
infibulation (n.): 1640s, noun of action from infibulate (q.v.); perhaps from or modeled on French infibulation or Medieval Latin *infibulatio.
infidel (n.): mid-15c., "adherent of a religion opposed to Christianity," from Middle French infidèle, from Latin infidelis "unfaithful, not to be trusted," in Late Latin "unbelieving" (in Medieval Latin also as a noun, "unbeliever"), from in- "not, opposite of" (see in- (1)) + fidelis "faithful" (see fidelity).

Originally "a non-Christian" (especially a Saracen); later "one who does not believe in religion, disbeliever in religion generally" (1520s). Also used to translate Arabic qafir (see kaffir), 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). As an adjective from mid-15c., "of a religion opposed to Christianity;" 1520s as "rejecting the Christian religion while accepting no other."
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 (12c.) or directly from Latin infidelitatem (nominative infidelitas) "unfaithfulness, faithlessness," noun of quality from infidelis "unfaithful, unbelieving" (see infidel).
infield (n.): 1733, "land of a farm which lies nearest the homestead," from in (adv.) + field (n.). Baseball diamond sense first attested 1866. Related: Infielder (1867).
infiltrate (v.): 1758, of fluids, from in- (2) "in" + filtrate (v.). Perhaps modeled on French infiltrer (16c.). Military sense of "penetrate enemy lines" attested from 1934. Related: Infiltrated; infiltrating.
infiltration (n.): "action or process of infiltrating," in physics, 1796, noun of action from infiltrate. Figurative sense of "a passing into" (anything immaterial) is from 1840; military sense of "stealthy penetration of enemy lines" dates from 1930. The same word had been used earlier in a medical sense of "a knitting together" (early 15c.), from Medieval Latin infiltratio.
infinite (adj.): late 14c., "eternal, limitless," also "extremely great in number," from Old French infinit "endless, boundless" and directly from Latin infinitus "unbounded, unlimited, countless, numberless," from in- "not, opposite of" (see in- (1)) + finitus "defining, definite," from finis "end" (see finish (v.)). 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, less than any assignable quantity," from Modern Latin infinitesimus, from Latin infinitus "infinite" (see infinite) + ordinal word-forming element -esimus, as in centesimus "hundredth." Related: Infinitesimally.
infinitive (n.): "simple, uninflected form of a verb, expressing its general sense," 1510s, from earlier use as an adjective (mid-15c.), from Late Latin infinitivus "unlimited, indefinite," from Latin infinitus "not limited" (see infinite). "Indefinite" because not restricted by person or number. Related: Infinitival; infinitively.
infinitude (n.): 1640s, from Medieval Latin *infinitudo, from Latin infinitus (see infinite) on model of multitudo, magnitudo. Or the English word is perhaps from or modeled on French infinitude (1610s).
infinity (n.): late 14c., from Old French infinité "infinity; very large number or quantity" (13c.), from Latin infinitatem (nominative infinitas) "boundlessness, endlessness," from infinitus boundless, unlimited" (see infinite). Latin 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, not strong or firm" (figuratively "superstitious, pusillanimous, inconstant"), from in- "not, opposite of" (see in- (1)) + firmus "strong; stable," figuratively "constant, trusty" (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," formerly also enfermerie, also firmary, fermery, from Old French enfermerie "hospital" and directly from Medieval Latin infirmaria "a place for the infirm," from Latin infirmus "weak, frail," (see infirm). According to OED, 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," also "the weaker sex" [Lewis], noun of quality from infirmus "weak, frail" (see infirm). Perhaps in part from Middle French infirmité, Old French enfermete "illness, sickness, disease; moral weakness."
inflame (v.): mid-14c., "make (someone) ardent; set (the spirit, etc.) on fire" with a passion or religious virtue, a figurative sense, from Old French enflamer "catch fire; set on fire" (Modern French enflammer), from Latin inflammare "to set on fire, kindle," figuratively "to rouse, excite," from in- "in" (see in- (2)) + flammare "to flame," from flamma "a flame" (see flame (n.)).

The literal sense of "to cause to burn" first recorded in English late 14c. Meaning "to heat, make hot, cause inflammation" is from 1520s. Formerly also enflame, but since 16c. the spelling with in- has predominated. Related: Inflamed; Inflaming.
inflammable (adj.): "able to be set alight," c. 1600, from Middle French inflammable, from Medieval Latin inflammabilis, from Latin inflammare "to set on fire" (see inflame).Since 1980s use of the word, especially in safety warnings, has been sometimes discouraged for fear it could be misunderstood as meaning "non-flammable" through confusion of the two prefixes in-. The word was used earlier in medicine in the sense "liable to inflammation" (early 15c.). Related: Inflammability.
inflammation (n.): early 15c., in pathology, "excessive redness or swelling in a body part," from Old French inflammation (14c.) and directly from Latin inflammationem (nominative inflammatio) "a kindling, a setting on fire," noun of action from past participle stem of inflammare "to set on fire" (see inflame). Literal sense "act of setting on fire" in English is from 1560s.
inflammatory (adj.): "tending to rouse passions or desires," 1711, a figurative use from Latin inflammat-, past participle stem of inflammare "to set on fire" (see inflame) + -ory. From 1732 in pathology, "accompanied by (pathological) inflammation." as a noun from 1680s.
inflatable (adj.): 1821, from inflate + -able.
inflate (v.): early 15c., "cause to swell," from Latin inflatus (source also of Italian enfiare, Spanish inflar, French enfler), past participle of inflare "to blow into, inflate" (see inflation). Economics sense (of prices, currency, etc.) is from 1843. In some senses a back-formation from inflation. Related: Inflated; inflating.
inflation (n.): mid-14c., "swelling caused by gathering of 'wind' in the body; flatulence," also, figuratively, "outbursts of pride," from Latin inflationem (nominative inflatio) "a puffing up, a blowing into; flatulence," noun of action from past participle stem of inflare "blow into, puff up," figuratively "inspire, encourage," from in- "into" (see in- (2)) + flare "to blow" (see blow (v.1)).

Meaning "action of inflating with air or gas" is from c. 1600. 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, alter, influence," from in- "in" (see in- (1)) + flectere "to bend" (see flexible). Grammatical sense "to vary by change of form" (especially at the end of a word) is fromd 1660s. Related: Inflected; inflecting.
inflected (adj.): 1640s, "bent, curved," past-participle adjective from inflect (v.). Grammatical sense is from 1775.
inflection (n.): also inflexion, 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 "to bend in, to change" (see inflect). For spelling, see connection. Grammatical sense "variation by declension or conjugation" is from 1660s; pronunciation sense "modulation of the voice" is from c. 1600.
"Derivation" can be defined as the process by which lexical items belonging to different word-classes are drawn from given bases. Derivation must be distinguished from inflexion, by which different paradigmatic forms are created from given stems. Inflexion describes plural formations, forms of comparison, etc. Inflexion processes do not change the word-class to which the lexical item under consideration belongs. [Alfred Bammesberger, "English Etymology," Heidelberg, Carl Winter, 1984]
inflexibility (n.): 1610s, from inflexible + -ity.
inflexible (adj.): late 14c., "incapable of being bent, physically rigid," also figuratively, "unyielding in temper or purpose," from Middle French inflexible and directly from Latin inflexibilis "that cannot be bent," from in- "not, opposite of" (see in- (1) + flexibilis "pliant, yielding" (see flexible). 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; also see -xion. Related: Inflexional.
inflict (v.): 1560s, "assail, trouble;" 1590s, "lay or impose as something that must be suffered," from Latin inflictus, past participle of infligere "to strike or dash against; inflict," from in- "in" (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, "act of inflicting;" 1580s, "that which is inflicted," from Middle French infliction (15c.), or directly from Late Latin inflictionem (nominative inflictio) "an inflicting, a striking against," noun of action from past participle stem of Latin infligere "to strike or dash against" (see inflict).
inflorescence (n.): 1760, "arrangement of flowers on a stem in relation to one another," 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 (v.)). Meaning "a beginning to bloom" in English is from 1800.
inflow (n.): 1839, from in (adj.) + flow (n.).
influence (n.): late 14c., an astrological term, "streaming ethereal power from the stars when in certain positions, 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, a flowing in," 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, stream in, pour in," from in- "into, in, on, upon" (see in- (2)) + fluere "to flow" (see fluent).

The range of senses in Middle English were non-personal, in reference to any outflowing of energy that produces effect, of fluid or vaporous substance as well as immaterial or unobservable forces. Meaning "exertion of unseen influence by persons" is from 1580s (a sense already in Medieval Latin, for instance Aquinas); meaning "capacity for producing effects by insensible or invisible means" is from 1650s. Under the influence (of alcohol, etc.) "drunk" first attested 1866.
influence (v.): 1650s, from influence (n.). Related: Influenced; influencing.
influent (adj.): mid-15c., "abundant, flowing in," in reference to occult power of the stars, etc., also of grace, from Latin influentem (nominative influens) "flowing in," present participle of influere "to flow in" (see influence (n.)). Also occasionally in the sense "influential" (1630s).