Online Etymology Dictionary

impertinence (n.): c. 1600, "incivility," from French impertinence, from impertinent (see impertinent). Meaning "irrelevance" is from 1620s. Impertinency is from 1580s as "a triviality, an absurdity."
impetigo (n.): pustular disease of the skin, late 14c., from Latin impetigo "skin eruption," from impetere "to attack" (see impetus). Originally used generally; the sense narrowed in modern times to specific diseases. Related: Impetiginous.
impersonator (n.): 1833, "one who embodies the person or character of another;" 1840 as "one who infuses (something) with a personality;" 1842 as "dramatic actor, one who plays a part on stage," from impersonate with Latinate agent noun suffix. Meaning "one who imitates the manners and speech of another" for entertainment (by 1921) perhaps grew from older theatrical use of female impersonator (1876), male impersonator (1874), both once popular stage acts; the first example of the former was perhaps Miss Ella Wesner, who had a vogue c. 1870.
Her impersonation were a genuine surprise and her success was so pronounced that in a short period a host of imitators made their appearance. Her most successful rivals were Bessie Bonehill, Millie Hilton and Vesta Tilley, all of London. [M.B Leavitt, "Fifty Years in Theatrical Management," New York, 1912]

There is no member of a minstrel company who gets a better salary than a good female impersonator, the line being considered a very delicate one, requiring a high style of art in its way to judge where fun stops and bad taste begins, with decision enough on the part of the performer to stop at the stopping place. ["The Ancestry of Brudder Bones," Harper's New Monthly Magazine, April 1879]
In Britain, blackface performers were called negro impersonators (1906). As a fem. formation, impersonatrix, as if from Latin, is from 1847; impersonatress, as if from French, is from 1881.
implausible (adj.): "not having an appearance of truth or credibility," 1670s, from assimilated form of in- "not, opposite of" (see in- (1)) + plausible. Earlier it meant "not worthy of applause" (c. 1600). Related: Implausibly.
implode (v.): 1870 (implied in imploded), back-formation from implosion. Related: Imploding.
importance (n.): "the quality of having consequence," c. 1500, from Middle French importance or directly from Medieval Latin importantia "importance," from importantem "important" (see important).
importunity (n.): "persistence, insistence; over-eagerness," early 15c., from Old French importunité (14c.), from Latin importunitatem (nominative importunitas) "unsuitableness; unmannerliness, unreasonableness, incivility," from importunus "unfit, troublesome" (see importune).
impossible (adj.): late 14c., from Old French impossible (14c.), from Latin impossibilis "not possible," from assimilated form of in- "not, opposite of" (see in- (1)) + possibilis (see possible). Weakened sense of "unable to be accomplished or tolerated" is from mid-19c. Related: Impossibly.
impost (n.): "tax, duty," 1560s, from Middle French impost (15c., Modern French impôt), from Medieval Latin impostum "a tax imposed," noun use of neuter of Latin impostus, contracted form of impositus, past participle of imponere "to place upon, impose upon" (see impostor). Compare depot. As an architectural term, 1660s, from French imposte (16c.), from Italian imposta, from the same Latin source.
imposture (n.): "act of willfully deceiving others," 1530s, from Middle French imposture or directly from Late Latin impostura "deceit," from impostus (see impost (n.)). Related: Imposturous.
impotence (n.): early 15c., "physical weakness," also "poverty," from Old French impotence "weakness" (13c.), from Latin impotentia "lack of control or power," from impotentem "lacking control, powerless" (see impotent). In reference to a complete want of (male) sexual potency, from c. 1500. The figurative senses of the word in Latin were "violence, fury, unbridled passion," via the notion of "want of self-restraining power," and these sometimes were used in English. Related: Impotency.
impoverish (v.): early 15c., empoverischen, from Old French empoveriss-, stem of empoverir, from em- + povre "poor" (see poor (adj.)). In the same sense Middle English also had empover (early 15c., from Old French enpoverir). Related: Impoverished; impoverishing.
impractical (adj.): 1823, from assimilated form of in- (1) "not, opposite of" + practical (adj.). Impracticable in the same sense dates from 1670s; unpractical is rare. Related: Impractically.
imprecate (v.): "call down by prayer" (typically of curses or malevolent desires), 1610s, probably a back-formation from imprecation. Related: Imprecated; imprecating; imprecatory (1580s).
impossibility (n.): late 14c., "quality of being impossible," from impossible + -ity; perhaps from or modeled on Old French impossibilité (14c.). Meaning "an impossible thing or occurrence" is from c. 1500. Sometimes in English 15c.-18c. it meant "inabolity, impotence," after a use of Medieval Latin impossibilitas.
imprecise (adj.): 1804, from assimilated form of in- (1) "not, opposite of" + precise. Earlier was unprecise (1756). Related: Imprecisely.
imprecision (n.): "inexactness," 1771, from assimilated form of in- (1) "not, opposite of" + precision.
impression (n.): late 14c., "mark produced by pressure," also "image produced in the mind or emotions by something external," from Old French impression "print, stamp; a pressing on the mind" (13c.), from Latin impressionem (nominative impressio) "a pressing into, onset, attack," figuratively "a perception, mental impression," literally "a pressing into," from imprimere "press into or upon" (see impress (v.1)).

Meaning "act or process of making a mark upon the surface by pressing" is from early 15c.. Meaning "copy made by pressure from type or an engraving" is from 1550s; that of "printing of a number of copies, aggregate of copies printed at one time" is from 1570s. Meaning "belief, vague notion" (as in under the impression) is from 1610s.
impressionism (n.): 1839 as a term in philosophy, from impression + -ism. With reference to the French art movement, 1879, from impressionist. Extended 1880s to music (Debussy), literature, etc.
improvement (n.): mid-15c., enprowment "profitable use, management of something for profit," from Anglo-French emprowement, from emprouwer "turn to profit" (see improve). Meaning "betterment; act of making better, amelioration" is from 1640s. Meaning "production of something better, something better (than something else)" is from 1712. Meaning "buildings, etc. on a piece of property" is from 1773. Related: Improvements.
imprison (v.): c. 1300, from Old French emprisoner "imprison; be in prison" (12c.), from assimilated form of in- "in" (see in- (2)) + prison (see prison). Formerly also emprison. Related: Imprisoned; imprisoning.
improbable (adj.): 1590s, from assimilated form of in- (1) "not, opposite of" + probable, or else from Latin improbabilis. Related: Improbably.
impure (adj.): mid-15c., of wine, "muddy, not clear," from Middle French impur (13c.), from Latin impurus "not pure, unclean, filthy, foul," from assimilated form of in- "not, opposite of" (see in- (1)) + purus "pure" (see pure).

In English, the subsequent order of sense appearance seems to be "earthly, mundane, not spiritual" (c. 1500); "obscene, lewd, unchaste, immoral" (1530s); "mixed with offensive matter, tainted" (1590s); "mixed or combined with other things" (without reference to foulness), 1620s. As a noun from 1784. Related: Impurely.
impurity (n.): mid-15c., "thing which makes or is impure;" c. 1500, "fact or quality of being impure," from Middle French impurité, from impur (see impure). Related: Impurities.
imputation (n.): 1540s, noun of action from impute (v.) on model of Middle French imputation, or else from Late Latin imputationem (nominative imputatio) "a charge, an account," noun of action from Latin imputare "to charge, ascribe."
infanticide (n.): 1650s, "the killing of infants," especially the killing of newborns or the unborn; 1670s, "one who kills an infant," from infant + -cide. Perhaps from French infanticide (16c.).
In Christian and Hebrew communities infanticide has always been regarded as not less criminal than any other kind of murder; but in most others, in both ancient and modern times, it has been practised and regarded as even excusable, and in some enjoined and legally performed, as in cases of congenital weakness or deformity among some of the communities of ancient Greece. [Century Dictionary]
inanition (n.): in medicine, "exhaustion from lack of nourishment," c. 1400, "pathological draining or depletion of blood, humors, or bodily fluids," from Old French inanition (14c.) and directly from Latin inanitionem (nominative inanitio) "emptiness," noun of action from past participle stem of inanire "to empty," from inanis "empty, void; worthless, useless," a word of uncertain origin.
inamorato (n.): "male lover; man who is in love," 1590s, from Italian innamorato, noun use of masc. past participle of innamorare "to fall in love" (see inamorata).
incarcerate (v.): "imprison, shut up in jail," 1550s, a back-formation from incarceration (q.v.), or else from Medieval Latin incarceratus, past participle of incarcerare "to imprison." Related: Incarcerated; incarcerating.
incidental (adj.): "casual, occurring casually in connection with something else; of minor importance," 1640s, from Medieval Latin incidentalis, from incidens (see incident (n.)). The earlier adjective in this sense was incident (1520s). Incidentals (n.) "'occasional' expenses, etc.," is attested by 1707. Incidental music "background music," originally in operas, is from 1812.
incinerator (n.): "device for waste disposal by burning," 1872, from incinerate + Latinate agent noun suffix -or.
incubate (v.): 1640s (transitive), "to brood upon, watch jealously" (figurative); 1721 in literal sense "to sit on (eggs) to hatch them," from Latin incubatus, past participle of incubare "to lie in or upon," also in the figurative sense "brood" (see incubation). Intransitive sense "to sit upon eggs" is from 1755. Related: Incubated; incubating.
incubator (n.): "apparatus for hatching eggs by artificial heat," 1845, agent noun from incubate (v.). Late Latin incubator meant "one who lies in a place."
indiscrete (adj.): "not containing distinct parts," 1782 (earlier "not distinctly separate," c. 1600), from Latin indiscretus "unseparated; indistinguishable, not known apart," from in- "not, opposite of" (see in- (1)) + discretus "separated, distinct" (in Medieval Latin "discerning, careful"), past participle of discernere "distinguish" (see discern). Related: Indiscretely; indiscreteness.
infamy (n.): early 15c., "public disgrace, dishonor, evil fame," from Old French infamie "dishonor, infamous person" (14c.) and directly from Latin infamia "ill fame, bad repute, dishonor," from infamis "disreputable, notorious, of ill fame," from in- "not, without" (see in- (1)) + fama "reputation" (see fame (n.)). Meaning "quality of being shamefully vile" is from 1510s.

An earlier form in Middle English was infame (late 14c.), from Old French infame, an earlier form of infamie. Infame also was the Middle English verb in this set, "brand with infamy," from Old French infamer, from Latin infamare "bring into ill repute, defame," from infamis. The verb has become archaic in English (infamize is attested from 1590s).
imbrue (v.): early 15c., "to soak, steep;" mid-15c., "to stain, soil," from Old French embruer "to moisten," which probably is a metathesis of embevrer "give to drink, make drunk," from em- (see em-) + -bevrer, ultimately from Latin bibere "to drink" (see imbibe). Or perhaps from Old French embroue "soiled," ultimately from boue "mud, dirt."
imbricate (v.): "to lay one over another" (as shingles, etc.), 1704 (implied in imbricated), from Latin imbricatus "covered with tiles," past participle of imbricare "to cover with rain tiles" (see imbrication). As an adjective from 1650s. Related: Imbricated; imbricating.
imburse (v.): "supply with money, store up," literally "put in a purse," 1520s, from Medieval Latin imbursare, from assimilated form of in- "into, in, on, upon" (see in- (2)) + bursa "purse" (see bursa). Related: Imbursement.
imbecilic (adj.): 1875, from the noun imbecile + -ic.
imbalance (n.): 1895, from im- "not" + balance (n.).
imago (n.): "final or adult stage of an insect," 1797, from Latin imago "an image, a likeless" (see image (n.)). "The name is due to the fact that such an insect, having passed through its larval stages, and having, as it were, cast off its mask or disguise, has become a true representation or image of its species." [Century Dictionary]
immunodeficiency (n.): 1969, from comb. form of immune + deficiency.
immunology (n.): by 1906, a hybrid from immune + -ology. Related: Immunological; immunologist.
immodest (adj.): 1560s, "arrogant, impudent, not modest about one's pretentions," from Latin immodestus "unrestrained, excessive," from assimilated form of in- "not, opposite of" (see in- (1)) + modestus (see modest). Meaning "indecent, lewd, not modest in person or utterance" is from 1580s. Related: immodestly.
immiscible (adj.): "incapable of being mixed" (as oil and water are), 1670s, from assimilated form of in- "not, opposite of" (see in- (1)) + miscible, from Latin miscere "to mix" (see mix (v.)).
immerge (v.): 1620s (trans.), "immerse, plunge into (a fluid)," from Latin immergere "to dip, plunge into" (see immersion). Intransitive sense from 1706. Rare; the usual verb is immerse. Related: Immerged; immerging.
immensity (n.): mid-15c., "vastness; infinity," from Middle French immensité (14c.) or directly from Latin immensitatem (nominative immensitas) "immeasurableness," noun of quality from immensus "immeasurable, boundless" (see immense). Immenseness is from c. 1600.
immeasurable (adj.): late 14c., immesurable, from im- + measurable. It could alternate with immensurable. Related: Immeasurably.
imitable (adj.): 1550s, from French imitable (16c.), from Latin imitabilis "that may be imitated," from imitari "to copy, portray" (see imitation). Related: Imitability.
il-: assimilated form of Latin prefix in- used with words beginning in l-; see in-.