Online Etymology Dictionary

implacable (adj.): "unappeasable," early 15c., from Old French implacable, from Latin implacabilis "unappeasable," from assimilated form of in- "not, opposite of" (see in- (1)) + placabilis "easily appeased" (see placate). Related: Implacably.
implant (v.): 1540s, "to plant in" (abstractly, of ideas, emotions, etc.), from Middle French implanter "to insert, engraft" (alongside Old French emplanter "to plant"), literally "plant in," from assimilated form of in- "into, in, on, upon" (see in- (2)) + planter "to plant" (see plant (n.)). Meaning "surgically implant (something) in the body" is from 1886, originally of teeth. Implanted is attested from early 15c., probably based on Medieval Latin implantus. Related: Implanting.
implement (n.): mid-15c., "supplementary payment, amount needed to complete repayment," from Late Latin implementem "a filling up" (as with provisions), from Latin implere "to fill, fill up, make full; fatten; fulfill, satisfy," from assimilated form of in- "into, in, on, upon" (see in- (2)) + plere "to fill" (see pleio-). Sense of "workman's tool, utensil of a trade, things necessary to do work" is 1530s. The underlying connection of the senses is "whatever may supply a want, that which fills up a need." Related: Implemental; implements.
implication (n.): early 15c., "action of entangling," from Latin implicationem (nominative implicatio) "an interweaving, an entanglement," noun of state from past participle stem of implicare "involve, entangle; embrace; connect closely, associate," from assimilated form of in- "into, in, on, upon" (see in- (2)) + plicare "to fold" (see ply (v.1)). Meaning "that which is implied (but not expressed), inference drawn from what is observed" is from 1550s.
implicit (adj.): 1590s, "implied, resting on inference," from Middle French implicite and directly from Latin implicitus, later variant of implicatus "entangled, confused, involved," past participle of implicare "entangle, involve" (see implication). From c. 1600 as "resulting from perfect confidence (in authority), unquestioning" (especially of faith).
implore (v.): c. 1500, from Middle French implorer and directly from Latin implorare "call on for help, beseech, beg earnestly," with a literal sense probably of "plead tearfully, invoke with weeping," from assimilated form of in- "on, upon" (see in- (2)) + plorare "to weep, cry out," a word of unknown origin. Related: Implored; imploring; imploringly; imploration.
implosion (n.): "a bursting inward, a sudden collapse," 1829, modeled on explosion, with assimilated form of in- "into, in, on, upon" (see in- (2)).
And to show how entire the neglect and confusion have been, they speak in the same breath of all these explosions, and of the explosion of a mixture of hydrogen and oxygen, the result of which, instead of being a gas or an enlargement of bulk, a positive quantity, is a negative one. It is a vacuum, in a popular sense, because the produce is water. The result is an implosion (to coin a word), not an explosion .... ["Gas-light," "Westminster Review," October 1829]
In early use often in reference to effect of deep sea pressures, or in phonetics. Figurative sense is by 1960.
impolite (adj.): 1610s, "unrefined, rough," from Latin impolitus "unpolished, rough, inelegant, unrefined," from assimilated form of in- "not, opposite of" (see in- (1)) + politus "polished" (see polite). Sense of "discourteous, uncivil, unpolished in manners" is from 1739. Related: Impolitely.
import (v.): early 15c., "signify, show, bear or convey in meaning," from Latin importare "bring in, convey, bring in from abroad," from assimilated form of in- "into, in" (see in- (2)) + portare "to carry" (see port (n.1)). In English, the sense of "bring from another state or land," especially "bring in goods from abroad" first recorded 1540s. As "be important" from 1580s. Related: Imported; importing.
important (adj.): mid-15c., from Middle French important and directly from Medieval Latin importantem (nominative importans) "important, momentous," present-participle adjective from importare "be significant in," from Latin importare "bring in" (see import (v.)). Meaning "pretentious, pompous" is from 1713. Related: Importantly.
importune (v.): "harass with solicitation, demand persistently," 1520s, back-formation from importunity, or else from Middle French importuner, from Medieval Latin importunari "to make oneself troublesome," from Latin importunus "unfit, unfavorable, troublesome," literally "having no harbor" (thus "difficult to access"), from assimilated form of in- "not, opposite of" (see in- (1)) + portus "harbor" (see port (n.1)). Related: Importuned; importuning. As an adjective from early 15c. Portunus was the Roman deity of harbours; hence Portunium "temple of Portunus."
impose (v.): late 14c., "to lay (a crime, duty, obligation, etc.) to the account of," from Old French imposer "put, place; impute, charge, accuse" (c. 1300), from assimilated form of in- "into, in" (see in- (2)) + poser "put, place" (see pose (v.1)). From c. 1500 as "apply authoritatively." Sense of "lay on as a burden, inflict by force or authority" first recorded 1580s. Related: Imposed; imposer; imposing.
imposition (n.): late 14c., "a tax, duty, tribute," from Old French imposicion "tax, duty; a fixing" (early 14c.), from Latin impositionem (nominative impositio) "a laying on," noun of action from past participle stem of imponere "to place upon," from assimilated form of in- "into, in" (see in- (2)) + ponere "to put, place" (past participle positus; see position (n.)). Sense of "the act of putting (something) on (something else)" is from 1590s. Meaning "an act or instance of imposing" (on someone) first recorded 1630s (see impose).
impostor (n.): 1580s, "swindler, cheat," from Middle French imposteur (16c.), from Late Latin impostor "a deceiver," agent noun from impostus, contraction of impositus, past participle of imponere "place upon, impose upon, deceive," from assimilated form of in- "into, in, on, upon" (see in- (2)) + ponere "to put place" (past participle positus; see position (n.)). Meaning "one who passes himself off as another" is from 1620s. Related: Impostrous. For a fem. form, Bacon uses French-based impostress (1610s) while Fuller, the church historian, uses Latinate impostrix (1650s).
impotent (adj.): late 14c., "physically weak, enfeebled, crippled," from Old French impotent "powerless, weak, incapable of doing," from Latin imponentem (nominative impotens) "lacking control, powerless, feeble; lacking self-control," from assimilated form of in- "not, opposite of" (see in- (1)) + ponentem (nominative potens) "potent" (see potent).

Meaning "having no power to accomplish anything" is from mid-15c.; that of "completely lacking in sexual power" (of males) is from mid-15c. Middle English also had a native term for this: Cunt-beaten (mid-15c.). The figurative sense in Latin was "without self-control, headstrong, violent, ungovernable, lacking self-restraint," which sometimes is found in English (OED cites examples from Spenser, Massinger, Dryden, and Pope). Related: Impotently.
impound (v.): early 15c., "to shut up in a pen or pound," from assimilated form of in- "into, in" (see in- (2)) + pound (n.). Originally of cattle seized by law. Related: Impounded; impounding.
impracticable (adj.): "incapable of being done, not to be done by available means," 1670s, from assimilated form of in- (1) "not, opposite of" + practicable. Earlier in a sense of "impassable" (1650s). Related: Impracticably; impracticability.
imprecation (n.): mid-15c., "a curse, cursing," from Latin imprecationem (nominative imprecatio) "an invoking of evil," noun of action from past participle stem of imprecari "invoke, pray, call down upon," from assimilated form of in- "into, in, within" (see in- (2)) + precari "to pray, ask, beg, request" (see pray). "Current limited sense is characteristic of human nature" [Weekley].
impregnable (adj.): early 15c., imprenable "impossible to capture," from Old French imprenable "invulnerable," from assimilated form of in- "not, opposite of" (see in- (1)) + Old French prenable "assailable, vulnerable" (see pregnable). With restored -g- from 16c. Related: Impregnably.
impregnate (v.): c. 1600, "to fill with an ingredient, spirit, etc.;" 1640s as "make (a female) pregnant), from Late Latin impraegnatus "pregnant," past participle of impraegnare "to render pregnant," from assimilated form of in- "into, in" (see in- (2)) + praegnare "make pregnant" (see pregnant). Earlier in same sense was impregn (1530s), which OED marks as "now only in poetic use."
impresario (n.): "one who organizes public entertainments," 1746, from Italian impresario "operatic manager," literally "undertaker (of a business)," from impresa "undertaking, enterprise, attempt," fem. of impreso, past participle of imprendere "undertake," from Vulgar Latin imprendere, from assimilated form of in- "into, in, on, onto" (see in- (2)) + prehendere "to grasp" (see prehensile).
imprescriptible (adj.): "inalienable, not subject to law or convention," 1560s, from French imprescriptible (16c.) or a native formation from assimilated form of in- (1) "not, opposite of" + Latin praescriptus, past participle of praescribere "to write beforehand, determine in advance, ordain, dictate" (see prescribe). Usually with right (n.). Related: Imperscriptibility. Alternative imprescribable is attested from 1887.
impress (v.1): late 14c., "have a strong effect on the mind or heart, to stamp deeply in the mind," from Latin impressus, past participle of imprimere "press into or upon, stamp," also figurative, from assimilated form of in- "into, in, on, upon" (see in- (2)) + premere "to press" (see press (v.1)). Literal sense of "to apply with pressure, make a permanent image in, indent, imprint" is from early 15c. in English. Related: Impressed; impressing.
impressionist adj., n.: in reference to a style of painting aiming to represent overall impressions as they first strike the eye rather than exact details, 1876 (adjective and noun), from French, coined 1874 by French critic Louis Leroy ("école impressionniste") in a disparaging reference to Monet's sunset painting "Impression, Soleil Levant." Later extended to other arts.
imprint (v.): formerly also emprint, late 14c., imprenten, emprenten, "to mark by pressure, stamp; to impress on the mind or memory," from Old French empreinter "to stamp, engrave, imprint," from empreinte "mark, impression, imprint" (13c.), noun use of fem. past participle of eimpreindre "to impress, imprint," from Vulgar Latin *impremere, from Latin imprimere "to impress, imprint" (see impress (v.1)).
impromptu (adv.): 1660s, from French impromptu (1650s), from Latin in promptu "in readiness," from assimilated form of in- "into, in, on, upon" (see in- (2)) + promptu, ablative of promptus "ready, prepared; set forth, brought forward," from past participle of promere "to bring out," from pro- "before, forward, for" (see pro-) + emere "to obtain" (see exempt (adj.)). From 1764 as an adjective; as a noun from 1680s.
improper (adj.): mid-15c., "not true," from Old French impropre (14c.) and directly from Latin improprius "not proper," from assimilated form of in- "not, opposite of" (see in- (1)) + proprius (see proper). Meaning "not suited, unfit" is from 1560s; that of "not in accordance with good manners, modesty, or decency" is from 1739. Related: Improperly (late 14c.).
improve (v.): late 15c., "to use to one's profit, to increase (income)," from Anglo-French emprouwer "to turn to profit" (late 13c.), from Old French en-, a causative prefix or from em-, + prou "profit," from Latin prode "advantageous" (see proud (adj.)).

Spelling with -v- was rare before 17c.; it apparently arose from confusion of -v- and -u-. Spelling otherwise deformed by influence of words in -prove. Meaning "make better, raise to a better quality or condition" first recorded 1610s. Intransitive sense "get better" is from 1727. Phrase improve the occasion retains the etymological sense. Meaning "to turn land to profit" (by clearing it, erecting buildings, etc.) was in Anglo-French (13c.) and survived or was revived in the American colonies and Australia. Hence, "make good use of, occupy (a place) and convert to some purpose."
improvident (adj.): 1510s, from assimilated form of in- (1) "not, opposite of" + provident. It retains a stronger connection with the "provide" aspect of Latin providere than provident nowdoes. Related: Improvidently.
improvisation (n.): "act of improvising musically," 1786, from French improvisation, from improviser "compose or say extemporaneously" (17c.), from Italian improvisare "to sing or speak extempore," from Latin improviso "unforeseen; not studied or prepared beforehand," ablative of improvisus "not foreseen, unexpected," from assimilated form of in- "not, opposite of" (see in- (1)) + provisus "foreseen," also "provided," past participle of providere "foresee, provide" (see provide). From music the sense expanded to a general meaning "do or perform on the spur of the moment."
impudent (adj.): late 14c., from Latin impudentem (nominative impudens) "without shame, shameless," from assimilated form of in- "not, opposite of" (see in- (1)) + pudens "ashamed, modest," present-participle adjective from pudere "to cause shame" (see pudendum). Related: Impudently.
impugn (v.): "attack by argument," late 14c., from Old French impugner (14c.), from Latin impugnare "to fight against, assault, attack," from assimilated form of in- "into, in, on, upon" (see in- (2)) + pugnare "to fight" (see pugnacious). Related: Impugned; impugning. Impugnable has meant "liable to be assailed" (1823) and "that cannot be assailed" (1560s).
impulse (n.): early 15c., "an act of impelling, a thrust, push," from Latin impulsus "a push against, pressure, shock," figuratively "incitement, instigation," past participle of impellere "to strike against, push against" (see impel). Meaning "a stimulus in the mind to action, arising from some state or feeling" is first recorded 1640s. As an adjective, in reference to purchases made on impulse, 1955 (in impulse buyer).
impunity (n.): 1530s, from Middle French impunité (14c.) and directly from Latin impunitatem (nominative impunitas) "freedom from punishment, omission of punishment," also "rashness, inconsideration," from impunis "unpunished, without punishment," from assimilated form of in- "not, opposite of" (see in- (1)) + poena "punishment" (see penal).
impute (v.): early 15c., from Old French imputer, emputer (14c.) and directly from Latin imputare "to reckon, make account of, charge, ascribe," from assimilated form of in- "in, into" (see in- (2)) + putare "to trim, prune; reckon, clear up, settle (an account)," from PIE *puto- "cut, struck," suffixed form of root *pau- (2) "to cut, strike, stamp" (see pave). Related: Imputed; imputing.
jack-in-the-box (n.): also jack-in-a-box, 1560s, a name for a sharp or cheat, "who deceived tradesmen by substituting empty boxes for others full of money" [Robert Nares, "A Glossary of Words, Phrases, Names, and Allusions," London, 1905]. See Jack + box (n.1). As a type of toy involving a figure on a spring inside a box, it is attested by 1702. Also it has been used variously to mean "peddler who sells wares from a temporary stall" (1690s), "an unborn child", a type of gambling game, a hermit crab, a large wooden male screw, the sacrament, and various mechanical devices.
in- (1): word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne "not" (see un- (1)).

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
in- (2): element meaning "into, in, on, upon" (also im-, il-, ir- by assimilation of -n- with following consonant), from Latin in- "in" (see in).

In Old French (and hence in Middle English) this often became en-, which in English had a strong tendency to revert to Latin in-, but not always, which accounts for pairs such as enquire/inquire. There was a native form, which in West Saxon usually appeared as on- (as in Old English onliehtan "to enlighten"), and some of those verbs survived into Middle English (such as inwrite "to inscribe"), but all now seem to be extinct.

Not related to in- (1) "not," which also was a common prefix in Latin, causing confusion: to the Romans impressus could mean "pressed" or "unpressed;" inaudire meant "to hear," but inauditus meant "unheard of;" in Late Latin investigabilis could mean "that may be searched into" or "that cannot be searched into." Latin invocatus was "uncalled, uninvited," but invocare was "to call, appeal to."

The trouble has continued in English; the hesitation over what is meant by inflammable being a commonly cited example. Implume (1610s) meant "to feather," but implumed (c. 1600) meant "unfeathered." Impliable can mean "capable of being implied" (1865) or "inflexible" (1734). Impartible in 17c. could mean "incapable of being divided" or "capable of being imparted." Impassionate can be "free from passion" or it can mean "strongly stirred by passion." Inanimate (adj.) is "lifeless," but Donne uses inanimate (v.) to mean "infuse with life or vigor." Irruption is "a breaking in," but irruptible is "unbreakable." In addition to improve "use to one's profit," Middle English also had a verb improve meaning "to disprove" (15c.). To inculpate is "to accuse," but inculpable means "not culpable, free from blame." Infestive has meant "troublesome, annoying" (1560s, from infest) and "not festive" (1620s). In Middle English inflexible could mean "incapable of being bent" or "capable of being swayed or moved." In 17c., informed could mean "current in information," formed, animated," or "unformed, formless" ("This was an awkward use" [OED]). Inhabited has meant "dwelt in" (1560s) and "uninhabited" (1610s); inhabitable likewise has been used on opposite senses, a confusion that goes back to Late Latin.
inamorata (n.): "female lover, woman with whom one is in love," 1650s, from Italian innamorata "mistress, sweetheart," noun use of fem. of innamorato, past participle of innamorare "to fall in love," from in "in" (from Latin, see in) + amore "love," from Latin amor (see Amy).
inane (adj.): 1660s, "empty, void," from Latin inanis or else a back-formation from inanity (q.v.). Sense of "silly, empty-headed" is from 1819. Related: Inanely. Bailey's Dictionary (1731) has inaniloquent "given to empty talk."
indignation (n.): c. 1200, from Old French indignacion "fury, rage; disrespect," or directly from Latin indignationem (nominative indignatio) "indignation, displeasure; a provocation, cause for indignation," noun of action from past participle stem of indignari "regard as unworthy, be angry or displeased at," from indignus "unworthy," from in- "not, opposite of" (see in- (1)) + dignus "worthy" (see dignity). The indignation meeting (1835) once was a common American way to express popular outrage by passing and publishing resolutions.
indigo (n.): 17c. spelling change of indico (1550s), "blue powder obtained from certain plants and used as a dye," from Spanish indico, Portuguese endego, and Dutch (via Portuguese) indigo, all from Latin indicum "indigo," from Greek indikon "blue dye from India," literally "Indian (substance)," neuter of indikos "Indian," from India (see India).

Replaced Middle English ynde (late 13c., from Old French inde "indigo; blue, violet" (13c.), from Latin indicum). Earlier name in Mediterranean languages was annil, anil (see aniline). As "the color of indigo" from 1620s. As the name of the violet-blue color of the spectrum, 1704 (Newton).
indirect (adj.): late 14c., from Old French indirect (14c.) or directly from Late Latin indirectus "not direct," from in- "not, opposite of" (see in- (1)) + directus (see direct (adj.)). Related: Indirectness.
indiscreet (adj.): "imprudent, not discrete, lacking good judgment," early 15c., from Medieval Latin indiscretus, from in- "not, opposite of" (see in- (1)) + Latin discretus "separated, distinct" (in Medieval Latin "discerning, careful"), past participle of discernere "distinguish" (see discern). A Medieval Latin secondary sense of the word that also became indiscrete. Related: Indiscreetly; indiscreetness.
indiscriminate (adj.): "not carefully discriminating, done without making distinctions," 1640s, from in- (1) "not, opposite of" + discriminate (adj.).
indispensable (adj.): 1530s, "not subject to dispensation," from Medieval Latin *indispensabilis, from in- "not, opposite of" (see in- (1)) + dispensabilis, from Latin dispensare "disburse, administer, distribute (by weight)" (see dispense). Meaning "necessary" is from 1690s. From 17c. into 19c. often spelled indispensible, but modern dictionaries considered this improper.

As a noun, "indispensable thing," from 1794; c. 1800-1810, after French use, it was the name of a type of pocket bag worn by women. indispensables (1820) also was one of the many 1820s jocular euphemisms for "trousers" (see inexpressible). Related: Indispensably.
indisposed (adj.): c. 1400, "unprepared;" early 15c., "not in order," from in- (1) "not" + disposed; or else from Late Latin indispositus "without order, confused." From mid-15c. in English as "diseased;" modern sense of "not very well, slightly ill" is from 1590s. A verb indispose is attested from 1650s but perhaps is a back-formation of this, rather than its source, or from French indisposer.
individual (adj.): early 15c., "one and indivisible, inseparable" (with reference to the Trinity), from Medieval Latin individualis, from Latin individuus "indivisible," from in- "not, opposite of" (see in- (1)) + dividuus "divisible," from dividere "divide" (see divide). Original sense now obsolete; the word was not common before c. 1600 and the 15c. example might be an outlier. Sense of "single, separate, of but one person or thing" is from 1610s; meaning "intended for one person" is from 1889.
indivisible (adj.): early 15c., from Old French indivisible (14c.) and directly from Late Latin indivisibilis "not divisible," from in- "not, opposite of" (see in- (1)) + divisibilis (see divisible). Related: Indivisibly.
Indo-China: also Indochina, "Further India, the region between India and China," 1815, from Indo- "India" + China. The name was said to have been proposed by Scottish poet and orientalist John Leyden, who lived and worked in India from 1803 till his death at 35 in 1811. French Indo-Chine is attested from 1813, but the source credits it to Leyden. The inappropriateness of the name was noticed from the start. Related: Indo-Chinese (1814).