Online Etymology Dictionary

indoctrinate (v.): formerly also endoctrinate, 1620s, "to teach," formed as if from Latin (but there seems to have been no word *indoctrinare), perhaps modeled on French endoctriner or extended from earlier (now obsolete) verb indoctrine, endoctrine, "to instruct" (mid-15c.); see in- (2) "in" + doctrine + -ate (2)). Meaning "to imbue with an idea or opinion" first recorded 1832. Related: Indoctrinated; indoctrinating.
Indo-European: 1814, coined by English polymath Thomas Young (1773-1829) and first used in an article in the "Quarterly Review," from Indo- + European. "Common to India and Europe," specifically in reference to the group of related languages and to the race or races characterized by their use.

The alternative Indo-Germanic (1835) was coined in German in 1823 (indogermanisch), based on the two peoples then thought to be at the extremes of the geographic area covered by the languages, but this was before Celtic was realized also to be an Indo-European language. After this was proved, many German scholars switched to Indo-European as more accurate, but Indo-Germanic continued in use (popularized by the titles of major works) and the predominance of German scholarship in this field made it the popular term in England, too, through the 19c. See also Aryan. Indo-Aryan (1850) seems to have been used only of the Aryans of India. Indo-European also was used in reference to trade between Europe and India or European colonial enterprises in India (1813).
indolence (n.): c. 1600, "indifference to pain," from French indolence (16c.) or directly from Late Latin indolentia "freedom from pain, insensibility," noun of state from Latin indolentem (nominative indolens) "insensitive to pain," from in- "not, opposite of, without" (see in- (1)) + dolentem (nominative dolens) "grieving," present participle of dolere "suffer pain, grieve" (see doleful). Originally of prisoners under torture, etc. The intermediate sense "state of rest or ease neither pleasant nor painful" (1650s) is now obsolete as well; main modern sense of "laziness, love of ease" (1710) perhaps reflects the notion of avoiding trouble (compare taking pains "working hard, striving (to do)").
The Castle hight of Indolence,
And its false Luxury;
Where for a little Time, alas!
We liv'd right jollity.

[Thomson, "The Castle of Indolence," 1748]
indomitable (adj.): 1630s, "that cannot be tamed or subdued," from Late Latin indomitabilis "untameable," from in- "not, opposite of, without" (see in- (1)) + *domitabilis, from Latin domitare, frequentative of domare "to tame" (see tame (adj.)). In reference to persons or personal qualities, "unyielding, persistent, resolute," by 1830. Related: Indomitably.
Indonesia: "the East Indies," 1850, from Indo- "India" + Greek nesos "island" (see Chersonese) + -ia. Formerly called Indian Archipelago or East Indies Islands (see Indies). Related: Indonesian "of or from the East Indies" (1850).
indoor (adj.): also in-door, 1711, opposed to outdoor, contracted from within door; the form indoors is attested from 1759 (within-doors is from 1750); as an adverb from 1801.
indri (n.): 1839, European name for the babakoto, a lemur-like arboreal primate of Madagascar (Indris Lichanotus); the common story since late 19c. is that the name was given in error by French naturalist Pierre Sonnerat (1748-1814), c. 1780, from mistaken use of Malagasy indry! "Look! See!" this being what his native guides said when they spotted the creature and called his attention to it.
However, as Hacking (1981) pointed out, Sonnerat was far too familiar with indris -- he described and figured them in detail, and apparently kept at least one in captivity -- for this story to be plausible. Furthermore, endrina is actually recorded as a native name for the indri (Cousins, 1885), and indri could easily be a variant of this name. Although the word endrina is first recorded in Malagasy only in 1835, there is no evidence that it could be a back-formation from the French indri (Hacking, 1981), and it seems implausible that the Malagasy would adopt an erroneous French name for an animal they were them selves familiar with. [Dunkel, Alexander R., et al., "Giant rabbits, marmosets, and British comedies: etymology of lemur names, part 1," in "Lemur News," vol. 16, 2011-2012, p.67]
indubitable (adj.): mid-15c., "too plain to admit of doubt," from Latin indubitabilis "that cannot be doubted," from in- "not, opposite of" (see in- (1)) + dubitabilis "doubtful," from dubitare "hesitate, doubt" (see doubt (v.)).
induce (v.): formerly also enduce, late 14c., "to lead by persuasions or other influences," from Latin inducere "lead into, bring in, introduce, conduct; persuade; suppose, imagine," from in- "into, in, on, upon" (see in- (2)) + ducere "to lead" (see duke (n.)). Meaning "to bring about" in any way (in reference to a trance, a fever, etc.) is from early 15c.; sense of "to infer by reasoning" is from 1560s. Electro-magnetic sense first recorded 1777. Related: Induced; inducing.
induct (v.): late 14c., "introduce, initiate, especially into office or employment," from Latin inductus, past participle of inducere "to lead into, introduce" (see induce). Originally of church offices; sense of "draft into military service" is 1917 in American English. Related: Inducted; inducting.
indulgence (n.): mid-14c., in the Church sense, "a freeing from temporal punishment for sin, remission from punishment for sin that remains due after absolution," from Old French indulgence or directly from Latin indulgentia "complaisance, a yielding; fondness, tenderness, affection; remission," from indulgentem (nominative indulgens) "indulgent, kind, tender, fond," present participle of indulgere "be kind; yield, concede, be complaisant; give oneself up to, be addicted," a word of uncertain origin; perhaps from in- "in" + a derivative of PIE root *dlegh- "to engage oneself," the source of play (v.) and plight (v.).

Sense of "leniency, forbearance of restraint or control of another, gratification of desire or humor" is attested from late 14c. That of "yielding to one's inclinations" (technically self-indulgence) in English is from 1630s. In British history, Indulgence also refers to grants of certain liberties to Nonconformists under Charles II and James II, as special favors rather than legal rights. The sale of indulgences in the original Church sense was done at times merely to raise money and was widely considered corrupt; the one in 1517 helped to spark the Protestant revolt in Germany.
industry (n.): late 15c., "cleverness, skill," from Old French industrie "activity; aptitude, experience" (14c.) or directly from Latin industria "diligence, activity, zeal," noun use of fem. of industrius "active, diligent," from early Latin indostruus "diligent," from indu "in, within" (see indigenous) + stem of struere "to build" (see structure (n.)). The meaning "habitual diligence, effort" is from 1530s; that of "systematic work" is from 1610s. The sense "a particular trade or manufacture" is first recorded 1560s.
inebriate (v.): late 15c., from Latin inebriatus, past participle of inebriare "to make drunk," from in- "in" (see in- (2)) + ebriare "make drunk," from ebrius "drunk," probably from PIE root *hegwh- "to drink." Related: Inebriated; inebriating. Also used in 19c. English were inebriacy (1842); inebriant, noun (1808) and adjective (1828); inebriety (1801); and inebrious (1711).
inedible (adj.): "unfit to eat," 1774, from in- (1) "not, opposite of" + edible. Related: Inedibly; inedibility (1879).
ineffable (adj.): late 14c., "beyond expression, too great for words, inexpressible," from Old French ineffable (14c.) or directly from Latin ineffabilis "unutterable," from in- "not, opposite of" (see in- (1)) + effabilis "speakable," from effari "utter," from ex- "out" (see ex-) + fari "to say, speak," from PIE root *bha- (2) "to speak, tell, say" (see fame (n.)). Meaning "that may not be spoken" is from 1590s. Plural noun ineffables was, for a time, a jocular euphemism for "trousers" (1823; see inexpressible). Related: Ineffably.
ineffectual (adj.): early 15c., from in- (1) "not, opposite of" + effectual. Related: Ineffectually; ineffectuality.
inefficacy (n.): "want of force or virtue to produce the desired effect," 1610s, from Late Latin inefficacia, from inefficacem (nominative inefficax), from in- "not, opposite of" (see in- (1)) + Latin efficax "powerful, effectual, efficient" (see efficacy).
inefficient (adj.): 1748, "not producing or incapable of producing the desired effect," from in- (1) "not, opposite of" + efficient. Related: Inefficiently.
ineligible (adj.): 1763, from in- (1) "not, opposite of" + eligible. Perhaps modeled on French inéligible. Related: Ineligibility.
ineluctable (adj.): "not to be escaped by struggling," 1620s, from French inéluctable (15c.) or directly from Latin ineluctabilis "unavoidable, inevitable," from in- "not, opposite of" (see in- (1)) + eluctabilis "that may be escaped from," from eluctari "to struggle out of," from ex- "out" (see ex-) + luctari "to struggle" (see reluctance).
inept (adj.): c. 1600, "not fit or suitable, inapt," also "absurd, foolish," from French inepte "incapable" (14c.) or directly from Latin ineptus "unsuitable, improper, impertinent; absurd, awkward, silly, tactless," from in- "not, opposite of" (see in- (1)) + aptus "apt" (see apt). Related: Ineptly; ineptness.
inequality (n.): early 15c., "difference of rank or dignity," from Old French inequalité (14c., Modern French inégalité) and directly from Medieval Latin inaequalitas, from Latin inaequalis "unequal, unlike, different (in size); changeable, inconstant," from in- "not, opposite of" (see in- (1)) + aequalis "equal" (see equal). In reference to magnitude, number, intensity, etc., from 1530s.
inerrant (adj.): 1650s, in reference to "fixed" stars (as opposed to "wandering" planets), from Latin inerrantem (nominative inerrans) "not wandering, fixed (of stars)," from in- "not, opposite of" (see in- (1)) + errans, present participle of errare "to wander, stray, roam, rove" (see err). Meaning "unerring, free from error" is from 1785.
inert (adj.): 1640s, "without inherent force, having no power to act or respond," from French inerte (16c.) or directly from Latin inertem (nominative iners) "unskilled, incompetent; inactive, helpless, weak, sluggish; worthless," used of stagnant fluids, uncultivated pastures, expressionless eyes. It is a compound of in- "without, not, opposite of" (see in- (1)) + ars (genitive artis) "skill" (see art (n.)). In chemistry, "having no active properties, neutral" (1800), specifically from 1885 of certain chemically inactive, colorless, odorless gases. Of persons or creatures, "indisposed or unable to move or act," from 1774.
inertia (n.): 1713, "that property of matter by virtue of which it retains its state of rest or of uniform rectilinear motion so long as no foreign cause changes that state" [Century Dictionary], introduced as a term in physics 17c. by German astronomer and physician Johann Kepler (1571-1630) as a special sense of Latin inertia "unskillfulness, ignorance; inactivity, idleness," from iners (genitive inertis) "unskilled; inactive" (see inert). Also sometimes vis inertia "force of inertia." Used in 1687 by Newton, writing in Modern Latin. The classical Latin sense of "apathy, passiveness, inactivity" is attested in English from 1822.
inevitable (adj.): "unavoidable," mid-15c., from Latin inevitabilis "unavoidable," from in- "not, opposite of" (see in- (1)) + evitabilis "avoidable," from evitare "to avoid," from ex- "out" (see ex-) + vitare "shun," originally "go out of the way." As a noun from 1850. Related: Inevitableness.
inexorable (adj.): "unyielding, unrelenting," 1550s, from Middle French inexorable and directly from Latin inexorabilis "that cannot be moved by entreaty, unyielding," from in- "not, opposite of" (see in- (1)) + exorabilis "able to be entreated," from exorare "to prevail upon," from ex- "out" (see ex-) + orare "pray" (see orator). Related: Inexorably; inexorability.
inexpedient (adj.): "not suitable to the purpose or circumstances," c. 1600, from in- (1) "not, opposite of" + expedient. Related: Inexpedience; inexpediently.
inexperience (n.): 1590s, from French inexpérience (15c.) or directly from Late Latin inexperientia "inexperience," from in- "not, opposite of" (see in- (1)) + Latin experientia "experimental knowledge; experiment; effort" (see experience (n.)).
inexplicable (adj.): early 15c., from Middle French inexplicable or directly from Latin inexplicabilis "that cannot be unfolded or disentangled, very intricate," figuratively, "inexplicable," from in- "not, opposite of" (see in- (1)) + explicabilis "that may be explained" (see explicable).

As a noun, 1745, "something that cannot be explained;" jocular inexplicables "trousers" is from 1829. Related: Inexplicably; inexplicability.
inextricable (adj.): early 15c., from Latin inextricabilis "that cannot be disentangled," from in- "not, opposite of" (see in- (1)) + extricare "to disentangle" (see extricate). Related: Inextricably; inextricability.
Inez: fem. proper name, Spanish form of Agnes (q.v.).
infallible (adj.): "exempt from error in judgment, knowledge, or opinion," early 15c., from Medieval Latin infallibilis, from in- "not, opposite of" (see in- (1)) + Late Latin fallibilis (see fallible). In reference to Popes, attested from 1870, hence infallibilism, the doctrine of this; infallibilist. Related: Infallibly.
infamous (adj.): a 16c. merger of two Middle English words, with the form of infamous "not well-known" (early 15c.) and the sense of infamis (late 14c.), "of ill repute, famous for badness." Infamous is from Medieval Latin infamosus, from in- "not, opposite of" (see in- (1)) + Latin famosus "celebrated" (see famous). Infamis is from Latin infamis "of ill fame" (see infamy).

Meaning "causing infamy" is from 1550s. As a legal term, "disqualified from certain rights of citizens because of conviction for certain crimes" (late 14c.). The neutral fameless (in the sense original to infamous) is recorded from 1590s. Related: Infamously.
infantry (n.): 1570s, from French infantrie, infanterie (16c.), from older Italian or Spanish infanteria "foot soldiers, force composed of those too inexperienced or low in rank to be cavalry," a collective noun from infante "foot soldier," originally "a youth," from Latin infantem (see infant). Meaning "infants collectively" is recorded from 1610s. A Middle English (c. 1200) word for "foot-soldiers" was going-folc, literally "going-folk."
infatuate (v.): 1530s, "turn (something) to foolishness, frustrate by making foolish," from Latin infatuatus, past participle of infatuare "make a fool of," from in- "in" (see in- (2)) + fatuus "foolish" (see fatuous). Specific sense of "inspire (in someone) a foolish passion beyond control of reason" is from 1620s. Related: Infatuated; infatuating.
infect (v.): late 14c., "fill with disease, render pestilential; pollute, contaminate; to corrupt morally," from Latin infectus, past participle of inficere "to stain, tinge, dye," also "to corrupt, stain, spoil," literally "to put in to, dip into," from in- "in" (see in- (2)) + facere "to make, do, perform" (see factitious). In Middle English occasionally in a neutral sense "tinge, darken," but typically used of things indifferent or bad, and especially of disease. Related: Infected; infecting.
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.
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.
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.
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).
in-fighting (n.): 1816, in pugilism, the practice of getting at close quarters with an opponent; see in + fighting. Old English infiht (n.) meant "brawl within a house or between members of a household." Middle English had infight (v.) "to attack" (c. 1300); the modern verb infight "fight at close quarters" (1916) appears to be a back-formation from in-fighting. Related: In-fighter (1812).
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.
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.
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.
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.