Online Etymology Dictionary

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.
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.
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.
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.
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.
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.
in-flight (adj.): also inflight, "during or within a flight," 1945, from in (prep.) + flight.
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.
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 in the astrological sense (see influence).
AN Article from Rome informs us that a Sort of Plague has broke out there, which destroys Abundance of their People, and they call it the Influenza. ["The Gentleman's Magazine," April 1743]
Used in Italian for diseases since at least 1504 (as in influenza di febbre scarlattina "scarlet fever") on notion of astral, occult, or atmospheric influence. The 1743 outbreak began in Italy. Often applied since mid-19c. to severe colds.
influx (n.): 1620s, from French influx (16c.) or directly from Late Latin influxus "a flowing in," from past participle stem of Latin influere "to flow in" (see influence (n.)). Originally of rivers, air, light, spiritual light, etc.; used of people from 1650s.
inform (v.): early 14c., "to train or instruct in some specific subject," from Old French informer, enformer "instruct, teach" (13c.) and directly from Latin informare "to shape, give form to, delineate," figuratively "train, instruct, educate," from in- "into" (see in- (2)) + formare "to form, shape," from forma "form" (see form (n.)). In early use also enform until c. 1600. Sense of "report facts or news, communicate information to" 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 (adj.). Meaning "irregular, unofficial, not according to rule or custom" is from c. 1600. Sense of "done without ceremony" is from 1828. Related: Informally.
information (n.): late 14c., informacion, "act of informing, communication of news," from Old French informacion, enformacion "advice, instruction," from Latin informationem (nominative informatio) "outline, concept, idea," noun of action from past participle stem of informare "to train, instruct, educate; shape, give form to" (see inform). The restored Latin spelling is from 16c.

Meaning "knowledge communicated concerning a particular topic" is from mid-15c. The word was used in reference to television broadcast signals from 1937; to punch-card operating systems from 1944; to DNA from 1953. Information theory is from 1950; information technology is from 1958 (coined in "Harvard Business Review"); information revolution, to be brought about by advances in computing, is from 1966. Information overload is by 1967.
infra-: word-forming element meaning "below, beneath," from Latin infra (adverb and preposition) "below, underneath, on the under side, beneath," also "later than; smaller than; inferior to," related to infernus "low, below," from PIE *ndher "under" (source also of Sanskrit adnah "below," Old English under "under, among;" see under). Modern popular use of it dates from the 1920s, as an opposite to super-, often in science fiction. "This use of infra- is scarcely a Latin one" [OED].
infraction (n.) $Look up infraction at Dictionary.com$: mid-15c., "the breaking of an agreement," from Old French infraction (13c.) and directly from Latin infractionem (nominative infractio) "a breaking, weakening," noun of action from past participle stem of infringere "to damage, break off, break, bruise," from in- "in" (see in- (2)) + frangere "to break" (see fraction). The verb infract (1560s) is archaic.
infra dig.: "beneath one's dignity, unbecoming to one's position in society," 1824, colloquial abbreviation of Latin infra dignitatem "beneath the dignity of." See infra- + dignity.
infralapsarian (adj.): 1731, from infra- + Latin lapsus "a fall" (see lapse (n.)) + ending from unitarian, etc.
[In theology], the doctrine held by Augustinians and by many Calvinists, that God planned the creation, permitted the fall, elected a chosen number, planned their redemption, and suffered the remainder to be eternally punished. The Sublapsarians believe that God did not permit but foresaw the fall, while the Supralapsarians hold that God not only permitted but decreed it. [Century Dictionary]
infra-red (adj.): also infrared, 1873, "below the red" (in the spectrum), from infra- + red (adj.1). As a noun, also from 1873.
infrastructure (n.): 1887, from French infrastructure (1875); see infra- + structure (n.). The installations that form the basis for any operation or system. Originally in a military sense.
infrequent (adj.): 1530s, "little used" (now obsolete); 1610s, "not occurring often," from Latin infrequentem (nominative infrequens) "occurring seldom, unusual; not crowded, absent," from in- "not" (see in- (1)) + frequens "repeated, regular, constant, often" (see frequent). Related: Infrequently.
infringe (v.): mid-15c., enfrangen, "to violate," from Latin infringere "to damage, break off, break, bruise," from in- "in" (see in- (2)) + frangere "to break" (see fraction). Meaning "encroach" first recorded c. 1760. Related: Infringed; infringing.
infuriate (v.): 1660s, from Italian infuriato, from Medieval Latin infuriatus, past participle of infuriare "to madden, enrage," from Latin in furia "in a fury," from ablative of furia (see fury). Also from 1660s as an adjective in English, but this use is rare. Related: Infuriated; infuriating.
infuse (v.): early 15c., "to pour in, introduce, soak (something in liquid)," from Latin infusus, past participle of infundere "to pour into, pour out; press in, crowd in; mix, mingle," from in- "in" (see in- (2)) + fundere "pour, spread" (see found (v.2)). Related: Infused; infusing; infusory; infusorial.
inarticulate (adj.): c. 1600, "not clear or intelligible" (of speech); "not jointed or hinged, not composed of segments connected by joints" (in biology), from Late Latin inarticulatus "not articulate, not distinct," from in- "not" (see in- (1)) + articulatus, past participle of articulare "to separate into joints; to utter distinctly" (see articulation). Of persons, "not able to speak clearly," 1754. Related: Inarticulately; inarticulateness; inarticulable.
inasmuch (adv.): contraction of phrase in as much "to such a degree," which is first attested c. 1300 as in als mikel, a Northern form. Contracted to in asmuch, then, beginning 14c. and especially since 17c., to one word. Meaning "in view of the fact" (followed by as) is from late 14c.
inauguration (n.): 1560s, from French inauguration "installation, consecration," and directly from Late Latin inaugurationem (nominative inauguratio) "consecration," presumably originally "installment under good omens;" noun of action from past participle stem of inaugurare "take omens from the flight of birds; consecrate or install when omens are favorable," from in- "on, in" (see in- (2)) + augurare "to act as an augur, predict" (see augur (n.)).
INAUGURATIO was in general the ceremony by which the augurs obtained, or endeavoured to obtain, the sanction of the gods to something which had been decreed by man; in particular, however, it was the ceremony by which things or persons were consecrated to the gods .... If the signs observed by the inaugurating priest were thought favourable, the decree of men had the sanction of the gods, and the inauguratio was completed. [William Smith (ed.), "Dictionary of Greek and Roman Antiquities," 1842]
inborn (adj.): Old English inboren "native to a place, indigenous, aboriginal," from in (prep.) "within" + boren "brought forth" (see born). Of personal qualities, "innate, implanted by nature," 1510s.
Inc.: U.S. abbreviation of Incorporated in company names (equivalent of British Ltd.), first attested 1904.
Inca (n.): 1590s, from Spanish Inga (1520s), from Quechea Inca, literally "lord, king." Technically only of the high Inca, but it was used widely among the Incas for "man of royal blood." Related: Incan.
incalculable (adj.): 1772, from in- (1) "not, opposite of" + calculable "that can be counted" (see calculate). Related: Incalculably; incalculability.
incandescent (adj.): 1794, from French incandescent (18c.) or directly from Latin incandescentem (nominative incandescens), present participle of incandescere "become warm, glow, kindle," from in- "within" (see in- (2)) + candescere "begin to glow, become white," inceptive of candere "to glow, to shine" (see candle). In reference to electric light, from 1881. The verb incandesce (1838), originally in science, is perhaps a back-formation.
incantation (n.): late 14c., from Old French incantacion "spell, exorcism" (13c.), from Late Latin incantationem (nominative incantatio) "art of enchanting," noun of action from past participle stem of incantare "to bewitch, charm, cast a spell upon, chant magic over, sing spells" (see enchantment).
incapable (adj.): 1590s, from Middle French incapable (16c.) and directly from Medieval Latin incapabilis "incapable," from in- "not" (see in- (1)) + capabilis "receptive; able to grasp or hold" (see capable). Related: Incapably; incapability.
incapacity (n.): 1610s, "lack of ability, powerlessness," from French incapacité (16c.), from Medieval Latin incapacitatem (nominative incapacitas), from Late Latin incapax (genitive incapacis) "incapable," from in- "not" (see in- (1)) + Latin capax "capable," literally "able to hold much," from capere "to take" (see capable). As a legal term (1640s), "lack of qualification," referring to inability to take, receive, or deal with in some way.
incarceration (n.): "fact of being imprisoned," 1530s, from Medieval Latin incarcerationem (nominative incarceratio), noun of action from past participle stem of incarcerare "to imprison," from in- "in" (see in- (2)) + carcer "prison, an enclosed space," from Proto-Italic *kar-kr(o)-, of uncertain origin.
It seems best to connect carcer with other IE words for 'circle, round object', such as [Latin] curvus, [Greek] κιρκος 'ring', [Old Norse] hringr, although not all of these have a good IE etymology. The reduplication in Latin carcer could be iconic; thus, the original meaning would have been 'enclosure'. [de Vaan]
The word appears earlier in English in an obsolete medical sense of "retention of pus" (early 15c.).
incarnadine: 1590s (adj.) "flesh-colored, carnation-colored, pale red, pink," from French incarnadin (16c.), from dialectal Italian incarnadino "flesh-color," from Late Latin incarnatio (see incarnation). The adjective now is archaic or obsolete. The word survives as a verb taken from the adjective, which properly would mean "to make flesh-colored," but means "make red" instead, the sense and the existence of the verb being entirely traceable to Lady Macbeth ("Macbeth" II ii.) in 1605. Its direct root might be the noun incarnadine "blood-red; flesh-color," though this is not attested until 1620s.
incarnation (n.): c. 1300, "embodiment of God in the person of Christ," from Old French incarnacion "the Incarnation" (12c.), from Late Latin incarnationem (nominative incarnatio), "act of being made flesh" (used by Church writers especially in reference to God in Christ; source also of Spanish encarnacion, Italian incarnazione), noun of action from past participle stem of Late Latin incarnari "be made flesh," from in- "in" (see in- (2)) + caro (genitive carnis) "flesh" (see carnage). Glossed in Old English as inflæscnes, inlichomung. As "person or thing that is the embodiment" (of some quality, deity, etc.) from 1742.
incendiary (n.): c. 1400, "person who sets malicious fires," from Latin incendiarius "an incendiary," literally "causing a fire" (see incendiary (adj.)). Meaning "person who enflames political passions" is from 1630s.
incense (n.): late 13c., "gum or other substance producing a sweet smell when burned," from Old French encens (12c.), from Late Latin incensum "burnt incense," literally "that which is burnt," noun use of neuter past participle of Latin incendere "set on fire" (see incendiary). Meaning "smoke or perfume of incense" is from late 14c.
incense (v.1): early 15c., encensen "to arouse, inspire," from Old French incenser, from Latin incensare, frequentative of incendere "set on fire," figuratively "incite, enrage, rouse" (see incendiary). From mid-15c. as "to provoke, anger." Literal sense "to heat, make (something) hot" is from c. 1500 in English but is rare.
incentive (n.): early 15c., "that which moves the mind or stirs the passion," from Late Latin incentivum, noun use of neuter of Latin adjective incentivus "setting the tune" (in Late Latin "inciting"), from past participle stem of incinere "strike up," from in- "in, into" (see in- (2)) + canere "to sing" (see chant (v.)). The sense apparently was influenced in Late Latin by association with incendere "to kindle." (Milton uses the adjective to mean "setting fire, incendiary.") Meaning "rewards meant to encourage harder work" is from 1948, short for incentive payment, etc. (see incentive (adj.)).
inception (n.): early 15c., "a beginning, a starting," from Old French inception and directly from Latin inceptionem (nominative inceptio) "a beginning; an undertaking," noun of action from past participle stem of incipere "begin, take in hand," from in- "in, on" (see in- (2)) + cipere comb. form of capere "take, seize" (see capable).
incessant (adj.): mid-15c., from Old French *incessant or directly from Late Latin incessantem (nominative incessans) "unceasing," from Latin in- "not" (see in- (1)) + cessans, present participle of cessare "to cease" (see cease (v.)). Related: Incessantly (early 15c.).
incest (n.): "the crime of sexual intercourse between near kindred," c. 1200, from Old French inceste "incest; lechery, fornication," and directly from Latin incestum "unchastity, impious unchastity," also specifically "sexual intercourse between close relatives," noun use of neuter adjective incestus "unchaste, impure," from in- "not" (see in- (1)) + castus "pure" (see caste). Old English had sibleger "incest," literally "kin-lying."
inch (n.1): "linear measure, one-twelfth of a foot," late Old English ynce, Middle English unche (current spelling c. 1300), from Latin uncia "a twelfth part," from unus "one" (see one). An early Anglo-Saxon borrowing from Latin; not found in other Germanic languages. Transferred and figurative sense of "a very small amount, small quantity" is attested from mid-14c. As the unit of measure of rainfall from 1845. Sometimes misdivided in Middle English as a neynche. Every inch "in every respect" is from early 15c. For phrase give him an inch ... see ell.
inch (n.2): "small Scottish island," early 15c., from Gaelic innis (genitive innse) "island," from Celtic *inissi (source also of Old Irish inis, Welsh ynys, Breton enez).
inchoate (adj.): "recently or just begun," 1530s, from Latin inchoatus, past participle of inchoare, alteration of incohare "commence, begin," probably originally "to hitch up," traditionally derived from in- "in" (see in- (2)) + a verb from cohum "strap (fastened to the oxen's yoke)," a word of obscure origin. De Vaan says that as, incohere "is a frequent verb, ... its meaning can easily have derived from 'to yoke a plough to a team of oxen' ..., in other words, 'to start work.' Thus, there might be a core of truth in the ancient connection of cohum with a yoke."
incident (n.): early 15c., "something which occurs casually in connection with something else," from Old French incident (13c.), and directly from Latin incidentem (nominative incidens), present participle of incidere "to fall in, fall, find the way; light upon, fall in with; fall upon, occur; happen, befall," from in- "on" (see in- (2)) + -cidere, comb. form of cadere "to fall" (see case (n.1)). Broader sense of "an occurrence viewed as a separate circumstance" is from mid-15c. Euphemistic meaning "event that might trigger a crisis or political unrest" first attested 1913.
incinerate (v.): "burn to ashes" (transitive), 1550s, from Medieval Latin incineratus, past participle of incinerare "reduce to ashes," from in- "into" (see in- (2)) + a verb from Latin cinis (genitive cineris) "ashes," from PIE root *keni- "dust, ashes" (source also of Greek konis "dust"). Middle English had the word, from Latin, but only as a past-participle adjective meaning "reduced to ashes" (early 15c.). Related: Incinerated; incinerating.
incipient (adj.): "beginning, commencing," 1660s, from Latin incipientem (nominative incipiens), present participle of incipere "begin, take up; have a beginning, originate," from in- "into, in, on, upon" (see in- (2)) + -cipere, comb. form of capere "to take" (see capable). Related: Incipiently.