Online Etymology Dictionary

incipit: opening word of a Latin book or manuscript, Latin, literally "(here) begins," third person singular present indicative of incipere "begin" (see incipient).
incision (n.): late 14c., "a cutting made in surgery," from Old French incision (13c.) and directly from Latin incisionem (nominative incisio) "a cutting into," recorded only in figurative senses, noun of action from past participle stem of incidere "to cut in," from in- "into, in, on, upon" (see in- (2)) + -cidere, comb. form of caedere "to cut" (see -cide). Meaning "act of cutting into" is from early 15c.
incisor (n.): "cutting tooth," 1670s, from Medieval Latin incisor "a cutting tooth," literally "that which cuts into," from Latin incisus, past participle of incidere "to cut into" (see incision). Inscisours as the name of a cutting tool is attested from early 15c. Related: Incisorial.
incite (v.): mid-15c., from Old French inciter, enciter "stir up, excite, instigate" (14c.), from Latin incitare "to put into rapid motion," figuratively "rouse, urge, encourage, stimulate," from in- "into, in, on, upon" (see in- (2)) + citare "move, excite" (see cite). Related: Incited; inciting.
inclement (adj.): 1660s, from French inclément (16c.) and directly from Latin inclementem (nominative inclemens) "harsh, unmerciful," from in- "not, opposite of, without" (see in- (1)) + clementem "mild, placid." "Limitation to weather is curious" [Weekley].
incline (v.): in early use also encline, c. 1300, "to bend or bow toward," from Old French encliner "to lean, bend, bow down," from Latin inclinare "to cause to lean; bend, incline, turn, divert," from in- "into, in, on, upon" (see in- (2)) + clinare "to bend," from PIE *klei-n-, suffixed form of *klei- "to lean" (see lean (v.)). Metaphoric sense of "have a mental disposition toward" is early 15c. in English (but existed in classical Latin). Related: Inclined; inclining.
include (v.): early 15c., "to shut (someone or something) in materially, enclose, imprison, confine," also "to have (something) as a constituent part," from Latin includere "to shut in, enclose, imprison, insert," from in- "in" (see in- (2)) + claudere "to shut" (see close (v.)). The alleged Sam Goldwyn-ism "Include me out" is attested from 1937. Related: Included; including.
incognito (adj./adv.): 1640s as both adjective ("disguised under an assumed name and character") and adverb ("unknown, with concealed identity"), from Italian incognito "unknown," especially in connection with traveling, from Latin incognitus "unknown, not investigated," from in- "not, opposite of, without" (see in- (1)) + cognitus, past participle of cognoscere "to get to know" (see cognizance). Also as a noun, "an unknown man" (1630s). Feminine form incognita was maintained through 19c. by those scrupulous about Latin. Incog was a common 18c. colloquial abbreviation.
incoherence (n.): 1610s, "want of coherence in thought or language," from in- (1) "not" + coherence; formed on model of Italian incoerenza. From 1670s in literal sense "want of physical coherence."
income (n.): c. 1300, "entrance, arrival," literally "a coming in;" see in (adv.) + come (v.). Perhaps a noun use of the late Old English verb incuman "come in, enter." Meaning "money made through business or labor" (i.e., "that which 'comes in' as payment for work or business") first recorded c. 1600. Compare German einkommen "income," Swedish inkomst. Income tax is from 1790, introduced in Britain during the Napoleonic wars, re-introduced 1842; in U.S. levied by the federal government 1861-72, authorized on a national level in 1913.
incommensurable (adj.): "having no common measure," 1550s, from Middle French incommensurable (14c.) or directly from Medieval Latin incommensurabilis, from in- "not, opposite of, without" (see in- (1)) + Late Latin commensurabilis, from Latin com- "with" (see com-) + mensurabilis "measurable," from mensurare "to measure" (see measure (v.)). Related: Incommensurably.
incommunicable (adj.): "not able to be communicated," 1570s, from in- (1) "not" + communicable. Earlier in a now-disused sense of "not communicative" (1570s). Related: Incommunicably.
incommunicado (adj./adv.): 1844, American English, from Spanish incomunicado, past participle of incomunicar "deprive of communication," from in- "not" (see in- (1)) + comunicar "communicate," from Latin communicare "to share, impart" (see communication).
incomparable (adj.): early 15c., from Old French incomparable (12c.) or directly from Latin incomparabilis "that cannot be equaled," from in- "not" (see in- (1)) + comparabilis "comparable" (see comparable). Related: Incomparably.
incompatible (adj.): "that cannot coexist or be conjoined," mid-15c., from Medieval Latin incompatibilis, from in- "not, opposite of" (see in- (1)) + compatibilis (see compatible). Originally of benefices, "incapable of being held together;" sense of "mutually intolerant" is from 1590s. Related: Incompatibly.
incompetent (adj.): 1610s, "insufficient," from French incompétent, from Late Latin incompetentem (nominative incompetens) "insufficient," from in- "not" (see in- (1)) + Latin competentem (see competent). Sense of "lacking qualification or ability" first recorded 1630s. The noun meaning "incompetent person" is from 1866. Related: Incompetently.
incomplete (adj.): late 14c., from Late Latin incompletus "incomplete," from in- "not" (see in- (1)) + completus (see complete (adj.)). Related: Incompletely; incompleteness.
incomprehensible (adj.): mid-14c., from Old French incomprehensible or directly from Latin incomprehensibilis "that cannot be seized or appreciated," from in- "not" (see in- (1)) + comprehensibilis (see comprehensible). Related: Incomprehensibly.
inconceivable (adj.): 1630s, from in- (1) "not, opposite of" + conceivable. Related: Inconceivably; inconceivability. An Old English word for this was unasmeagendlic.
incongruous (adj.): 1610s, from Latin incongruus "incongruous, inconsistent," from in- "not, opposite of, without" (see in- (1)) + congruus "fit, suitable" (see congruent). Related: Incongruously; incongruousness.
inconsequent (adj.): 1570s, "not following as a logical conclusion," from Latin inconsequentem (nominative inconsequens) "not logically connected, not resulting from what has preceded," from in- "not, opposite of, without" (see in- (1)) + consequens, past participle of consequi "to follow" (see consequence). Related: Inconsequently.
inconsiderate (adj.): late 15c., "done thoughtlessly, heedless, careless, indiscreet," from Latin inconsideratus "headstrong, unadvised," of persons, "thoughtless," literally "not properly considered," from in- "not, opposite of" (see in- (1)) + consideratus (see consider). Sense of "without regard for the feelings of others" is from 1842. Related: Inconsiderately.
inconsistent (adj.): 1640s, "not agreeing in substance or form;" 1650s, "self-contradictory," from in- (1) "not, opposite of" + consistent. Related: Inconsistently.
inconspicuous (adj.): 1620s, "invisible," from Late Latin inconspicuus "not conspicuous," from in- "not, opposite of, without" (see in- (1)) + Latin conspicuus (see conspicuous). Weakened sense of "not readily seen or noticed" developed by 1828. Related: Inconspicuously; inconspicuousness.
inconstant (adj.): c. 1400, "fickle, not steadfast," from Old French inconstant "variable, eccentric" (14c.), from Latin inconstantem (nominative inconstans) "changeable, fickle, capricious," from in- "not, opposite of, without" (see in- (1)) + constantem (see constant). Related: Inconstantly.
incontinent (adj.): late 14c., "wanting self-restraint," from Old French incontinent (14c.) or directly from Latin incontinentem (nominative incontinens) "immoderate, intemperate, not holding back," from in- "not, opposite of" (see in- (1)) + continens (see continent (adj.)). Originally chiefly of sexual appetites. General sense of "unable to retain" is from 1640s; medical sense of "unable to control bowels or bladder, unable to restrain natural discharges from the body" is attested by 1828.
He was incontynent, and with fleschely lustes he consumyd alle his tyme. ["Speculum Sacerdotale," 15th century]
inconvenient (adj.): late 14c., "injurious, dangerous," also "absurd, illogical" (senses now obsolete), from Latin inconvenientem (nominative inconveniens) "unsuitable, not accordant, dissimilar," from in- "not, opposite of" (see in- (1)) + convenientem (see convenient). In early 15c., "inappropriate, unbecoming, unnatural;" also, of an accused person, "unlikely as a culprit, innocent." Sense of "troublesome, incommodious, awkward" first recorded 1650s.
incorporate (v.): late 14c., "to put (something) into the body or substance of (something else), blend; absorb, eat," also "solidify, harden," often in medical writing, from Late Latin incorporatus, past participle of incorporare "unite into one body, embody, include," from Latin in- "into, in, on, upon" (see in- (2)) + verb from corpus (genitive corporis) "body" (see corporeal).

Meaning "to legally form a body politic with perpetual succession and power to act as one person, establish as a legal corporation" is from mid-15c. Intransitive sense of "unite with another body so as to become part of it" is from 1590s. Related: Incorporated; incorporating.
incorrect (adj.): early 15c., "uncorrected, not chastened into obedience," of sinners, etc. (a sense now obsolete), from Latin incorrectus "uncorrected, not revised," from in- "not" (see in- (1)) + correctus, past participle of corrigere "to put straight; to reform" (see correct (v.)). Sense of "not in good style" is from 1670s; that of "factually wrong, erroneous, inaccurate" is from 1750s (implied in incorrectly).
incorrigible (adj.): mid-14c., "incurable (of diseases, venom, etc.); extravagant (of expense); implacable (of hearts)," from Old French incorrigible "perfect, beyond rebuke or discipline" (14c.) or directly from Latin incorrigibilis "not to be corrected," from in- "not, opposite of" (see in- (1)) + corrigibilis, from corrigere "to correct" (see correct (v.)). From mid-15c. as "incapable of improvement" (of persons). Related: Incorrigibly. As a noun, from 1746.
increase (v.): mid-14c., encresen, "become greater in size or number" (intransitive); late 14c., "cause to grow, enlarge" (transitive), from Anglo-French encress-, Old French encreiss-, present participle stem of encreistre, from Latin increscere "to increase, to grow upon, grow over, swell, grow into," from in- "in" (see in- (2)) + crescere "to grow" (see crescent). Modern English restored the Latin spelling 16c. Related: Increased; increasing.
incredible (adj.): early 15c., "unbelievable, surpassing belief as to what is possible," from Latin incredibilis "not to be believed, extraordinary," from in- "not" (see in- (1)) + credibilis "worthy of belief" (see credible). Used c. 1400 in a now-extinct sense of "unbelieving, incredulous." Related: Incredibly; incredibility.
increment (n.): mid-15c., "act or process of increasing," from Latin incrementum "growth, increase; an addition," from stem of increscere "to grow in or upon" (see increase (v.)). Meaning "amount of increase" first attested 1630s.
incriminate (v.): 1730, back-formation from incrimination (q.v.) or else from Medieval Latin incriminatus, past participle of incriminare "to incriminate, accuse." Related: Incriminated; incriminating.
incroyable (n.): 1796, from French incroyable, literally "incredible" (15c.), from in- "not, opposite of, without" (see in- (1)) + croire "to believe," from Latin credere (see credo). A name for the French fop or dandy of the period of the Directory (1795-99). Said to be so called from their extravagant dress, and also, according to OED, from a favorite expression among them ("C'est vraiment incroyable").
incubation (n.): 1610s, "a brooding," from Latin incubationem (nominative incubatio) "a laying upon eggs," noun of action from past participle stem of incubare "to hatch," literally "to lie on, rest on," from in- "on" (see in- (2)) + cubare "to lie" (see cubicle). The literal sense of "sitting on eggs to hatch them" in English is first recorded 1640s.
incubus (n.): "imaginary being or demon, credited with causing nightmares, and, in male form, consorting with women in their sleep," c. 1200, from Late Latin incubus (Augustine), from Latin incubo "nightmare, one who lies down on (the sleeper)," from incubare "to lie upon" (see incubate). Plural is incubi. Compare succubus.
inculcate (v.): 1540s, from Latin inculcatus, past participle of inculcare "force upon, insist; stamp in, impress, tread down," from in- "in" (see in- (2)) + calcare "to tread, press in," from calx (1) "heel" (see calcaneus). Related: Inculcated; inculcating.
inculpate (v.): "to accuse, bring charges against," 1794, from Medieval Latin inculpatus, past participle of inculpare "to reproach, blame, censure," from Latin in- "in" (see in- (2)) + culpare "to blame," from culpa "fault." But inculpable (late 15c.) means "not culpable, free from blame," from Latin in- "not" (see in- (1)) + culpare.
incumbent (n.): early 15c., "person holding a church position," from Medieval Latin incumbentem (nominative incumbens) "holder of a church position," noun use of present participle of incumbere "to obtain or possess," from Latin incumbere "recline on," figuratively "apply oneself to," from in- "on" (see in- (2)) + -cumbere "lie down," related to cubare "to lie" (see cubicle). Extended to holders of any office from 1670s.
incur (v.): c. 1400, "bring (an undesirable consequence) upon oneself;" mid-15c. as "become liable for (payment or expenses)," from Anglo-French encurir, Old French encorir "to run, flee; commit, contract, incur" (Modern French encourir), from Latin incurrere "run into or against, rush at, make an attack;" figuratively, "to befall, happen, occur to," from in- "upon" (see in- (2)) + currere "to run" (see current (adj.)). Related: Incurred; incurring.
incurable (adj.): mid-14c., from Old French incurable "not curable" (13c.), from Late Latin incurabilis "not curable," from in- "not" (see in- (1)) + curabilis "curable" (see curable). As a noun, "incurable person," from 1650s. Related: Incurably.
incursion (n.): "hostile attack," early 15c., from Old French incursion "invasion, attack, assault" (14c.) or directly from Latin incursionem (nominative incursio) "a running against, hostile attack," noun of action from past participle stem of incurrere "run into or against, rush at" (see incur).
indebted (adj.): late 14c., endetted "owing money, liable for borrowed money," past participle of endetten "to indebt, oblige," from Old French endeter "to involve in debt, run into debt," from en- "in" (see in- (2)) + dete "debt" (see debt). Figurative sense of "under obligation for favors or services" first attested 1560s. Spelling re-Latinized in English from 16c. The verb indebt is now rare or obsolete. Related: Indebtedness. Latin indebitus meant "not owed, not due."
indecent (adj.): 1560s, "unbecoming, in bad taste," from French indécent (14c.) or directly from Latin indecentem (nominative indecens) "unbecoming, unseemly," from in- "not, opposite of, without" (see in- (1)) + decens (see decent). Sense of "offending against propriety" is from 1610s. Indecent assault (1861) originally covered sexual assaults other than rape or intended rape, but by 1934 it was being used as a euphemism for "rape." Related: Indecently
indecision (n.): 1735, from in- (1) "not, opposite of, without" + decision. Perhaps from or modeled on French indécision (17c.), which Cotgrave's "French and English Dictionary" (1673) translates with An undecision.
indecorous (adj.): 1680s, "in bad taste," from Latin indecorus "unbecoming, unseemly, unsightly; disgraceful," from in- "not, opposite of, without" (see in- (1)) + decorus "becoming, fitting, seemly, proper" (see decorous). Related: Indecorously; indecorousness (1670s).
indeed (adv.): c. 1600, a contraction into one word of the prepositional phrase in dede "in fact, in truth" (early 14c.), from Old English dæd "a doing, act, action, event" (see deed (n.)). As an interjection, 1590s; as an expression of surprise or disgust, 1834. Emphatic form yes (or no) indeedy attested from 1856, American English.
indefatigable (adj.): 1580s (implied in indefatigably), from French indefatigable (15c.), from Latin indefatigabilis "that cannot be wearied," from in- "not" (see in- (1)) + defatigare "to tire out," from de- "utterly, down, away" (see de-) + fatigare "to weary" (see fatigue (n.)).

Blount's "Glossographia" (1656) has defatigable, which also was in use elsewhere in 17c., but the modern use of defatigable (1948) probably is a jocular back-formation from indefatigable.
indefensible (adj.): 1520s, "that cannot be maintained or justified by argument," from in- (1) "not, opposite of" + defensible. From 1560s as "that cannot be maintained by force." Related: Indefensibly.