Online Etymology Dictionary

indicator (n.): 1660s, "that which indicates or points out," from Late Latin indicator, agent noun from indicare "to point out, show" (see indication). As a finger muscle, from 1690s. As a steam-cylinder's pressure gauge, 1839. As a device on a motor vehicle to signal intention to change direction, 1932.
Indianapolis: city in Indiana, U.S., founded 1821, from Indiana + -polis.
Indiana: by 1765 in English, a name given to the region north of the Ohio River mid-18c. by French explorers or settlers; see Indian + Latin-derived place-name suffix -ana. Organized as a U.S. territory 1800, admitted as a state 1816. Related: Indianian (1784).
indeterminate (adj.): late 14c., from Late Latin indeterminatus "undefined, unlimited," from in- "not, opposite of" (see in- (1)) + determinatus, past participle of determinare "to enclose, bound, set limits to" (see determine). Related: Indeterminately.
indescript (adj.): "undescribed," 1831, from in- (1) "not, opposite of" + Latin descriptus, past participle of describere (see describe).
indescribable (adj.): 1726, from in- (1) "not, opposite of" + describable (see describe). Related: Indescribably; indescribability (1797). In same sense, Old English had unasecgendlic. Indescribables for "trousers" (1819) was colloquial in England for a generation or so.
We cannot omit here to state, that, some years since, we recollect a rumour in the gallery [of the House of Commons], that Madame de Staël was sitting, en habit d'homme, in a surtout and military indescribables, listening to the debate, under the protection of Sir J. Macintosh. ["Privileges of Women," in "Retrospective Review," London, 1824]
See inexpressible.
indignance (n.): 1580s, from indignant + -ance or else from Medieval Latin indignantia. Indignancy is attested from 1778.
indigene (adj.): 1590s, from French indigène (16c.), from Latin indigena "sprung from the land," as a noun, "a native," literally "in-born" (see indigenous). As a noun from 1660s.
indigency (n.): 1610s, from Latin indigentia "need, want" (see indigence).
indictive (adj.): "proclaimed," 1650s, from Late Latin indictivus "proclaimed," from Latin indicere "to declare publicly, proclaim, announce," from in- "in" (see in- (2)) + dicere "to speak, say, tell" (see diction).
indignity (n.): "unworthy treatment; act intended to lower the dignity of another," 1580s, from Latin indignitatem (nominative indignitas) "unworthiness, meanness, baseness," also "unworthy conduct, an outrage," noun of quality from indignus "unworthy" (see indignation). Related: Indignities.
indiscernible (adj.): 1630s, from in- (1) "not, opposite of" + discernible. Related: Indiscernibly; indiscernibility.
indiscipline (n.): "disorder, lack of discipline," 1783, from in- (1) "not, opposite of" + discipline (n.). Perhaps modeled on French indiscipline (18c.). Indisciplined as a past participle adjective is attested from c. 1400.
indiscretion (n.): mid-14c., "want of discretion, imprudence," from Old French indiscrecion "foolishness, imprudence" (12c.), from Late Latin indiscretionem (nominative indiscretio) "lack of discernment," from in- "not, opposite of" (see in- (1)) + discretionem "discernment, power to make distinctions" (see discretion). Meaning "indiscreet act" is from c. 1600.
indisposition (n.): early 15c., "unfavorable influence" (in astrology), mid-15c., "disinclination (to), state of being not disposed in mind," from in- (1) "not, opposite of" + disposition. Perhaps modeled on Old French indisposicion or Medieval Latin indispositio. Sense of "ill health, disorder of the mind or body" is from mid-15c. Other 15c. senses included "inclination to evil; wickedness," and "public disorder, lawlessness."
indisputable (adj.): 1550s, from Late Latin indisputabilis, from in- "not, opposite of, without" (see in- (1)) + disputabilis, from Latin disputare "to weigh, examine, discuss, argue, explain" (see dispute (v.)). Related: Indisputably.
indecipherable (adj.): 1802, from in- (1) "not" + decipherable (see decipher (v.)). Undecipherable is older. Related: Indecipherably; indecipherability.
indefeasible (adj.): "not to be set aside or overcome," 1530s (implied in indefeasibly), from in- (1) "not, opposite of" + defeasible (see defeasance).
indention (n.): 1763, formed irregularly from indent (v.1). It could be a useful word if it split with indentation the two senses (relating to marginal notches and to dents) of that word, but indention, too, is used in both.
indissoluble (adj.): mid-15c. (implied in indissolubly), from Latin indissolubilis "indestructible, that cannot be dissolved," from in- "not, opposite of" (see in- (1)) + dissolubilis, from dis- + solubilis (see soluble). Related: Indissolubly.
indistinct (adj.): 1580s, "not seen or heard clearly," from Latin indistinctus "not distinguishable, confused, obscure," from in- "not, opposite of" (see in- (1)) + distinctus (see distinct). From c. 1600 as "not clearly defined or distinguished." Indistinctly is attested from c. 1400 in an obsolete sense "equally, alike, indiscriminately." Related: Indistinctness.
indistinguishable (adj.): 1640s, "not clearly perceived;" 1650s, "incapable of being told apart," from in- (1) "not, opposite of" + distinguishable. Shakespeare's use of it (c. 1600) seems to mean "of indeterminate shape." Related: Indistinguishably.
Indian Ocean: first attested 1515 in Modern Latin (Oceanus Orientalis Indicus), named for India, which projects into it; earlier it was the Eastern Ocean, as opposed to the Western Ocean (Atlantic) before the Pacific was surmised.
inexplicit (adj.): 1775 (implied in inexplicitly), from in- (1) "not, opposite of" + explicit. Or else from Latin inexplicitus "not to be unfolded; unexplained." Related: Inexplicitly; inexplicitness.
inexpensive (adj.): 1670s, from in- (1) "not, opposite of" + expensive. Related: Inexpensively.
inexperienced (adj.): "lacking the knowledge or skill gained by experience," 1620s, past-participle adjective from inexperience.
infancy (n.): late 14c., "condition of babyhood," also "childhood, youth," from Anglo-French enfaunce and directly from Latin infantia "early childhood," from infantem "young child," literally "one unable to speak" (see infant). Restriction to the earliest months of life is a return to the etymological sense of the word but is a recent development in English. In old legal language it meant "condition of being a minor" and could mean any age up to 21.
infarct (n.): substance of an infarction, 1873, from medical Latin infarctus (variant of infartus), past participle of infarcire "to stuff into" (see infarction).
infarction (n.): 1680s, noun of action from Latin infarcire "to stuff into," from in- "into" (see in- (2)) + farcire "to stuff" (see farce).
Formerly applied in pathology to a variety of morbid local conditions; now usually restricted to certain conditions caused by a local fault in the circulation. [Century Dictionary, 1902]
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).
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."
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).
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).
infertility (n.): c. 1600, from French infertilité (16c.) or directly from Late Latin infertilitatem (nominative infertilitas), from infertilis "not fertile, unfruitful" (see infertile).
infibulation (n.): 1640s, noun of action from infibulate (q.v.); perhaps from or modeled on French infibulation or Medieval Latin *infibulatio.
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.
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.
inflexion (n.): see inflection; also see -xion. Related: Inflexional.
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]
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.
informatics (n.): 1967, translating Russian informatika (1966); see information + -ics.
info (n.): 1906, short for information.
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).
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).
inferential (adj.): 1650s, from Medieval Latin inferentia (see inference) + -al (1). Related: Inferentially.
infelicitous (adj.): "unhappy, unlucky," 1754, from in- (1) "not, opposite of" + felicitous. Earlier was infelicious (1590s). Related: infelicitously; infelicitousness.
infeasible (adj.): 1530s, from in- (1) "not, opposite of" + feasible.
inexact (adj.): 1791, from in- (1) "not, opposite of" + exact (adj.). Perhaps modeled on French inexact (18c.). Related: Inexactly.
inequitable (adj.): "unfair, unjust," 1660s, from in- (1) "not, opposite of" + equitable, which is ultimately from Latin aequus "even, just, equal." Related: Inequitably. The same formation in English has also meant "impassable on horses, unfit for riding over" (1620s), from Latin inequabilis, from equus "a horse" (see equine).
inefficacious (adj.): "not producing the desired effect," 1650s, from in- (1) "not, opposite of" + efficacious. Related: Inefficaciously; inefficaciousness (1640s).