Online Etymology Dictionary

imperative (adj.): 1520s, in grammar, from Late Latin imperativus "pertaining to a command," from imperat-, past participle stem of imperare "to command, requisition," from assimilated form of in- "into, in" (see in- (2)) + parare "prepare" (see pare).
imperative (n.): mid-15c., in grammar; later "something imperative" (c. 1600), from Old French imperatif in the grammatical sense (13c.) and directly from Late Latin imperativus (see imperative (adj.)). In philosophy from 1796.
imperator (n.): "absolute ruler," 1580s, from Latin imperator "commander-in-chief, leader, master," agent noun from stem of imperare "to command" (see imperative (adj.)). In the Roman republic, a holder of military command during active service, also a title bestowed on victorious generals; in the Roman Empire, the emperor as commander-in-chief of the armies. Related: Imperatorial.
imperceptibility (n.): 1670s, from imperceptible + -ity.
imperceptible (adj.): early 15c., from Middle French imperceptible (15c.), from Medieval Latin imperceptibilis, from assimilated form of in- "not, opposite of" (see in- (1)) + perceptibilis (see perceptible). Related: Imperceptibly; imperceptibility. OED marks imperceivable (1610s) as "Now rare."
imperception (n.): "want of perception," 1620s, from assimilated form of in- (1) "not, opposite of" + perception.
imperceptive (adj.): "not perceiving," 1660s, from assimilated form of in- (1) "not, opposite of" + perceptive.
imperfect (adj.): late 14c., imparfit, "sinful, immoral; not properly formed, not complete, immature; rudimentary, elementary," from Old French imparfait, from Latin imperfectus "unfinished, incomplete, immature," from assimilated form of in- "not, opposite of" (see in- (1)) + perfectus "finished, complete" (see perfect (adj.)). Altered mid-16c. to conform to Latin, along with perfect. Related: Imperfectly.
imperfection (n.): late 14c., "incompleteness, deficiency, lack," from Old French imperfeccion "defect; imperfect state" (12c.) and directly from Late Latin imperfectionem (nominative imperfectio) "imperfection," noun of action from Latin imperfectus "imperfect"(see imperfect). Meaning "an instance of being imperfect" is from early 15c.
imperforate (adj.): "having no perforation," 1670s, from assimilated form of in- (1) "not, opposite of" + perforate (adj.). Related: Imperforation (1650s).
imperial (adj.): late 14c., "having a commanding quality," from Old French imperial, emperial "imperial; princely, splendid; strong, powerful" (12c.), from Latin imperialis "of the empire or emperor," from imperium "empire" (see empire). Meaning "pertaining to an empire" (especially Rome's) is from late 14c.; by 1774 of Britain's. Meaning "of imposing size or excellence" is from 1731. Imperial presidency in a U.S. context traces to Arthur Schlesinger Jr.'s book on the Nixon administration (1974). Related: Imperially. The noun is from 1520s as "member of the emperor's party;" 1670s as the name of gold coins issued by various imperial authorities.
imperialism (n.): 1826, "advocacy of empire, devotion to imperial interests," originally in a Napoleonic context, also of Rome and of British foreign policy; from imperial + -ism. At times in British usage (and briefly in U.S.) with a neutral or positive sense relating to national interests or the spread of the benefits of Western civilization, but from the begining usually more or less a term of reproach. General sense of "one country's rule over another," first recorded 1878. Picked up disparagingly in Communist jargon by 1918.
It is the old story of 1798, when French republicanism sick of its own folly and misdeeds, became metamorphosed into imperialism, and consoled itself for its incapacity to found domestic freedom by putting an iron yoke upon Europe, and covering it with blood and battle-fields. [Francis Lloyd, "St. James's Magazine," January 1842]
imperialist (n.): c. 1600, "an adherent of an emperor or the imperial cause," such as the emperor of Germany (in the Thirty Years War), France, China, etc., probably modeled on French impérialiste (early 16c.); from imperial + -ist. The shift in meaning to "advocate of imperialism" (1893) came via the British Empire, which involved a worldwide colonial system. See imperialism. As a term of abuse in communist circles, attested by 1918. As an adjective by 1816.
imperialistic (adj.): 1872, from imperialist + -ic.
imperil (v.): 1590s, from assimilated form of in- "into, in" (see in- (2)) + peril. Formerly also emperil. Related: Imperiled; imperiling; imperilment.
imperious (adj.): 1540s, "of a dominating character," from Latin imperiosus "commanding, mighty, powerful," from imperium "empire, command" (see empire). Formerly also emperious. Meaning "imperial" is from 1580s. Related: Imperiously; imperiousness.
Imperious applies to the spirit or manner of the person ruling or giving a command, and of rule in general; imperative, to the nature of a command. An imperious person is determined to have his will obeyed; imperious rule is characterized by the haughty, overbearing, and determined nature of the ruler. [Century Dictionary]
imperishable (adj.): "not subject to destruction or decay," 1640s, from assimilated form of in- "not, opposite of" (see in- (1)) + perishable. Related: Imperishably
imperium (n.): "authority to command the national military forces," in extended use "an empire," 1650s, from Latin imperium "command, supreme authority, power" (see empire). Hence Latin phrase imperium in imperio "a state within a state."
impermanence (n.): 1796, from impermanent + -ence. Impermanency is from 1640s.
impermanent (adj.): 1650s, from assimilated form of in- "not, opposite of" (see in- (1)) + permanent.
impermeable (adj.): 1690s, from French imperméable or directly from Late Latin impermeabilis, from assimilated form of in- "not, opposite of" (see in- (1)) + permeabilis (see permeable). Related: Impermeability.
impermissible (adj.): 1814, from assimilated form of in- "not, opposite of" (see in- (1)) + permissible.
imperscriptible (adj.): "unrecorded, without written authority," 1792 (used almost exclusively with right (n.)), from French imperscriptible, from assimilated form of Latin in- "not, opposite of" (see in- (1)) + perscribere "to write down." Earliest in English in reference to the French evolution. Spelling imperscriptable attested from 1827.
impersonal (adj.): mid-15c., a grammatical term, from Late Latin impersonalis, from assimilated form of in- "not, opposite of" (see in- (1)) + personalis "personal" (see personal). Sense of "not connected with any person" is from 1620s; that of "not endowed with personality, having no conscious individuality" is from 1842. Related: impersonally.
impersonality (n.): 1769, from impersonal + -ity.
impersonate (v.): 1620s, "represent in bodily form," from assimilated form of Latin in- "into, in" (see in- (2)) + persona "person." Sense of "assume the person or character of" is first recorded 1715; earlier in that sense was personate (1610s). Related: Impersonated; impersonating.
impersonation (n.): 1800, "personification;" 1825 as "an acting of a part or character;" noun of action from impersonate (v.).
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.
impersuadable (adj.): 1763, from assimilated form of in- (1) "not, opposite of" + persuadable. [Earliest date in OED 2nd ed. print is a typo.]
impersuasible (adj.): 1570s, from assimilated form of in- (1) "not, opposite of" + persuasible (see persuadable).
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."
impertinent (adj.): late 14c., "unconnected, unrelated, not to the point" (now obsolete; OED's last citation is from Coleridge), from Old French impertinent (14c.) or directly from Late Latin impertinentem (nominative impertinens) "not belonging," literally "not to the point," from assimilated form of Latin in- "not, opposite of" (see in- (1)) + pertinens (see pertinent). Sense of "rudely bold, uncivil, offensively presumptuous" is from 1680s, from earlier sense of "not appropriate to the situation" (1580s), which probably is modeled on similar use in French, especially by Molière, from notion of meddling in what is beyond one's proper sphere.
Impertinent means forward, intrusive, generally from curiosity but sometimes with undesired advice, etc.; officious means forward to offer and undertake service where it is neither needed nor desired. A busybody may be either impertinent or officious, or both. [Century Dictionary]
impertinently (adv.): mid-15c., "not to the point, irrelevantly," from impertinent + -ly (2). Meaning "intrusively, presumptuously" is from 1640s.
imperturbable (adj.): c. 1500, from Middle French imperturbable (15c.) and directly from Late Latin imperturbabilis "that cannot be disturbed" (Augustine), from assimilated form of in- "not, opposite of" (see in- (1)) + *perturbabilis, from Latin perturbare "to confuse, disturb" (see perturb). Related: Imperturbably (1785); imperturbability (1831; earlier as a dictionary word); imperturbation.
imperturbed (adj.): 1721, from assimilated form of in- (1) "not" + perturbed. Related: Imperturbedly.
impervious (adj.): 1640s, from Latin impervius "not to be traverse, that cannot be passed through, impassible," from assimilated form of in- "not, opposite of" (see in- (1)) + pervius "letting things through, that can be passed through," from per "through" (see per (prep.)) + via "road" (see via (adv.)). Related: Imperviously; imperviousness.
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.
impetuosity (n.): early 15c., "violent movement, rushing," from Old French impetuosité (13c.) and directly from Medieval Latin impetuositatem (nominative impetuositas), from Late Latin impetuosus "impetuous, violent" (see impetuous).
impetuous (adj.): late 14c., "hot-tempered, fierce;" late 15c., "done or given with a rush of force," from Old French impetuos (13c., Modern French impétueux) and directly from Late Latin impetuosus "impetuous, violent" (source also of Spanish and Italian impetuoso), from Latin impetus "attack" (see impetus). Related: Impetuously; impetuousness.
impetus (n.): early 15c., impetous "rapid movement, rush;" 1640s, with modern spelling, "force with which a body moves, driving force," from Latin impetus "an attack, assault; rapid motion; an impulse; violence, vigor, force;" figuratively "ardor, passion," from impetere "to attack," from assimilated form of in- "into, in, on, upon" (see in- (2)) + petere "aim for, rush at" (see petition (n.)).
impiety (n.): mid-14c., from Old French impieté "impiety, wickedness" (12c.) or directly from Latin impietatem (nominative impietas) "irreverence, ungodliness; disloyalty, treason," noun of quality from impius "irreverent" (see impious).
impinge (v.): 1530s, "fasten or fix forcibly," from Latin impingere "drive into, strike against," from assimilated form of in- "into, in, on, upon" (see in- (2)) + pangere "to fix, fasten" (see pact). Sense of "encroach, infringe" first recorded 1738. Related: Impinged; impinging; impingent.
impingement (n.): 1670s, "act of impinging;" see impinge + -ment.
impious (adj.): 1590s, "irreligious, lacking reverence for God," from Latin impius "without reverence, irreverent, wicked; undutiful, unpatriotic," from assimilated form of in- "not, opposite of" (see in- (1)) + pius (see pious). Related: Impiously; impiousness.
impish (adj.): 1650s, from imp + -ish. Related: Impishly; impishness.
implacability (n.): 1530s, from Late Latin implacabilitas, from Latin implacabilis "unappeasable" (see implacable).
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.
implant (n.): 1890, "thing implanted;" 1941 as "action of implanting," from implant (v.). Related: Implants, which is attested by 1981 as short for breast implants (1976).
implantation (n.): 1570s, "manner of being implanted," from French implantation, noun of action from implanter "to insert, engraft" (see implant (v.)). From c. 1600 as "act of implanting;" in embryology from 1902.