Online Etymology Dictionary

imagine (v.): mid-14c., "to form a mental image of," from Old French imaginer "sculpt, carve, paint; decorate, embellish" (13c.), from Latin imaginari "to form a mental picture, picture to oneself, imagine" (also, in Late Latin imaginare "to form an image of, represent"), from imago "an image, a likeness" (see image (n.)). Sense of "suppose, assume" is first recorded late 14c. Related: Imagined; imagining.
imam (n.): 1610s, from Arabic, literally "leader; one who precedes," from amma "to go before, precede." As a high religious title used differently by Sunni and Shiite, but also used of the leader of daily prayers in the mosque and generally for a Muslim prince or religious leader. Related: Imamate.
imbecility (n.): early 15c., "physical weakness, feebleness (of a body part), impotence," from Middle French imbécillité and directly from Latin imbecillitatem (nominative imbecillitas) "weakness, feebleness, helplessness," from imbecillus "weak, feeble," of uncertain origin. "Weakness in mind" (as opposed to body) was a secondary sense in Latin but was not attested in English until 1620s.

The Latin word traditionally is said to mean "unsupported" or "without a walking stick" (Isidore: quasi sine baculo), from assimilated form of in- "not, opposite of" (see in- (1)) + baculum "a stick" (see bacillus), but Century Dictionary finds that "improbable" and de Vaan adds "it seems to me that exactly the persons who can walk without a support are the stronger ones."
imbrication (n.): "an overlapping of edges" (as of roof tiles, etc.), 1640s, from French imbrication, noun of action from stem of Latin imbricare "to cover with tiles," from imbricem (nominative imbrex) "curved roof tile used to draw off rain," from imber (genitive imbris) "rain, heavy rain; rainwater," from PIE *ombh-ro- "rain" (source also of Sanskrit abhra "cloud, thunder-cloud, rainy weather," Greek ombros "rain, a shower"), from root *nebh- "moist; water" (see nebula).
imbroglio (n.): 1750, "a jumble;" 1818 as "complicated misunderstanding, intricate entanglement" (of persons, nations, etc.), from Italian imbroglio, from imbrogliare "confuse, tangle," from assimilated form of in- "into, in, on, upon" (see in- (2)) + brogliare "embroil," probably from Middle French brouiller "confuse" (see broil (v.2); also compare embroil).
imbue (v.): early 15c., "to keep wet; to soak, saturate;" also figuratively "to cause to absorb" (feelings, opinions, etc.), from Latin imbuere "moisten, wet, soak, saturate," figuratively "to fill; to taint," a word of uncertain origin, perhaps from the same root as imbrication. Compare also Old French embu, past participle of emboivre, from Latin imbibere "drink in, soak in" (see imbibe), which might have influenced the English word. Related: Imbued; imbuing.
imitation (n.): c. 1400, "emulation; act of copying," from Old French imitacion, from Latin imitationem (nominative imitatio) "a copying, imitation," noun of action from past participle stem of imitari "to copy, portray, imitate," from PIE *im-eto-, from root *aim- "copy" (source also of Hittite himma- "imitation, substitute;" Latin imago "image," aemulus "emulous"). Meaning "an artificial likeness" is from c. 1600. As an adjective, from 1840.
immaculate (adj.): mid-15c., "free from mental or moral pollution, pure," from a figurative use of Latin immaculatus "unstained," from assimilated form of in- "not, opposite of" (see in- (1)) + maculatus "spotted, defiled," past participle of maculare "to spot," from macula "spot, blemish," a word of uncertain origin. The literal sense of "spotlessly clean or neat" in English is first attested 1735. Related: Immaculately.

The phrase Immaculate Conception "freedom from original sin possessed by the Virgin Mary from her conception in her mother's womb" is from late 15c. in English (from Middle French conception immaculée); the idea itself had been debated in the Church since 12c., declared to be an article of faith in 1854.
immanent (adj.): "indwelling, remaining within, inherent," 1530s, via French immanent (14c.) or directly from Late Latin immanens, present participle of immanere "to dwell in, remain in," from assimilated form of in- "into, in, on, upon" (see in- (2)) + Latin manere "to dwell" (see manor). In medieval philosophy contrasted with transitive; later with transcendent. Related: Immanently.
immaterial (adj.): c. 1400, "spiritual, incorporeal, not consisting of matter," from Medieval Latin immaterialis "not consisting of matter, spiritual," from assimilated form of in- "not, opposite of" (see in- (1)) + Late Latin materialis "of or belonging to matter" (see material (adj.)). Sense of "unimportant, of no consequence" is first recorded 1690s from material (adj.) in its meaning "important" (16c.). Related: Immaterially (late 14c.); immateriality.
immature (adj.): 1540s, "untimely, premature," from Latin immaturus "untimely, unripe," from assimilated form of in- "not, opposite of" (see in- (1)) + maturus "ripe, timely, early" (see mature (v.)). In 16c., usually in reference to early death; main modern sense of "not fully developed" first recorded 1640s. In reference to mentalities or behaviors not considered age-appropriate, from 1920. Related: Immaturely.
immediate (adj.): late 14c., "intervening, interposed;" early 15c., "with nothing interposed; direct," also with reference to time, "without delay, instant," from Old French immediat (14c.), from Late Latin immediatus "without anything between," from assimilated form of in- "not, opposite of" (see in- (1)) + mediatus "in the middle" (see mediate).
immemorial (adj.): c. 1600, from French immémorial "old beyond memory" (16c.), from Medieval Latin immemorialis, from assimilated form of in- "not, opposite of" (see in- (1)) + Latin memorialis of or belonging to memory" (see memorial (n.)). Something immemorial is ancient beyond memory; something immemorable is not worth remembering. Latin immemor meant "unmindful, forgetful, heedless."
immense (adj.): "great beyond measure," early 15c., from Old French immense (mid-14c.), from Latin immensus "immeasurable, boundless," also used figuratively, from assimilated form of in- "not, opposite of" (see in- (1)) + mensus "measured," past participle of metiri "to measure" (see measure (v.)). A vogue word in 18c., and mocked as such:
For instance, a long while every thing was immense great and immense little, immense handsome and immense ugly. Miss Tippet from the cloisters, could not drink tea with Master Parchment at the White Conduit-house, unless it was an immense fine day, yet probably it might rain so immense, there was no going without a coach. ["Town and Country Magazine" (in "Annual Register" for 1772)]
immersion (n.): c. 1500, from Late Latin immersionem (nominative immersio), noun of action from past participle stem of immergere "to plunge in, dip into, sink, submerge," from assimilated form of in- "into, in, on, upon" (see in- (2)) + Latin mergere "plunge, dip" (see merge). Meaning "absorption in some interest or situation" is from 1640s. As a method of teaching a foreign language, 1965, trademarked by the Berlitz company.
immigrate (v.): "to pass into a place as a new inhabitant or resident," especially "to move to a country where one is not a native, for the purpose of settling permanently there," 1620s, from Latin immigratum, past participle of imigrare "to remove, go into, move in," from assimilated form of in- "into, in, on, upon" (see in- (2)) + migrare "to move" (see migration). Related: Immigrated; immigrating.
imminent (adj.): 1520s, from Middle French imminent (14c.) and directly from Latin imminentem (nominative imminens) "overhanging; impending," present participle of imminere "to overhang, lean towards," hence "be near to," also "threaten, menace, impend, be at hand, be about to happen," from assimilated form of in- "into, in, on, upon" (see in- (2)) + -minere "jut out," which is related to mons "hill" (see mount (n.1)). Related: Imminently.
immolate (v.): 1540s, "to sacrifice, kill as a victim," from Latin immolatus, past participle of immolare "to sacrifice," originally "to sprinkle with sacrificial meal," from assimilated form of in- "into, in, on, upon" (see in- (2)) + mola (salsa) "(sacrificial) meal," related to molere "to grind" (see mallet). Related: Immolated; immolating.
immortal (adj.): late 14c., "deathless," from Latin immortalis "deathless, undying" (of gods), "imperishable, endless" (of fame, love, work, etc.), from assimilated form of in- "not, opposite of" (see in- (1)) + mortalis "mortal" (see mortal (adj.)). In reference to fame, literature, etc., "unceasing, destined to endure forever, never to be forgotten, lasting a long time," attested from early 15c. (also in classical Latin). As a noun, "an immortal being," from 1680s.
immunity (n.): late 14c., "exemption from service or obligation," from Old French immunité "privilege; immunity from attack, inviolability" (14c.) and directly from Latin immunitatem (nominative immunitas) "exemption from performing public service or charge, privilege," from immunis "exempt, free," (see immune (adj.)). Medical sense of "protection from disease" is from 1879, from French or German.
immure (v.): 1580s, "enclose with walls, shut up, confine," from Middle French emmurer and directly from Medieval Latin immurare, literally "to shut up within walls," from assimilated form of in- "into, in" (see in- (2)) + Latin murus "wall" (see mural). Military sense of "fortify" is from 1590s. Related: Immured; immuring; immurement.
immutable (adj.): early 15c., "unchanging, unalterable," from Old French immutable (Modern French immuable), and directly from Latin immutabilis "unchangeable, unalterable," from assimilated form of in- "not, opposite of" (see in- (1)) + mutabilis "changeable," from mutare "to change" (see mutable). Related: Immutably.
imp (n.): Old English impe, impa "young shoot, graft," from impian "to graft," probably an early Germanic borrowing from Vulgar Latin *imptus, from Late Latin impotus "implanted," from Greek emphytos, verbal adjective formed from emphyein "implant," from em- "in" + phyein "to plant" (see physic). Compare Swedish ymp, Danish ympe "graft."

The sense of the word has shifted from plants to people, via the meaning "child, offspring" (late 14c., now obsolete), from the notion of "newness." The current meaning "little devil" is attested from 1580s, from common pejorative phrases such as imp of Satan. The extension from this to "mischievous or pert child" (1640s) unconsciously turns the word back toward its Middle English sense.
Suche appereth as aungelles, but in very dede they be ymps of serpentes. [Wynkyn de Worde, "The Pilgrimage of Perfection," 1526]
impact (v.): c. 1600, "press closely into something," from Latin impactus, past participle of impingere "to push into, dash against, thrust at" (see impinge). Original sense is preserved in impacted teeth. Sense of "strike forcefully against something" first recorded 1916. Figurative sense of "have a forceful effect on" is from 1935. Related: Impacting.
impair (v.): late 14c., a re-Latinizing of earlier ampayre, apeyre "make worse, cause to deteriorate" (c. 1300), from Old French empeirier "make worse" (Modern French empirer), from Vulgar Latin *impeiorare "make worse," from assimilated form of in- "into, in" (see in- (2)) + Late Latin peiorare "make worse" (see pejorative). In reference to driving under the influence of alcohol, first recorded 1951 in Canadian English. Related: Impaired; impairing.
impala (n.): 1875, from Zulu im-pala "gazelle."
impale (v.): 1520s, "to enclose with stakes, fence in" (a sense continued in specialized uses into 19c.), from Middle French empaler or directly from Medieval Latin impalare "to push onto a stake," from assimilated form of in- "into, in" (see in- (2)) + Latin palus "a stake, prop, stay; wooden post, pole," from PIE *pak-slo-, from root *pag-/*pak- "to fasten" (see pact). Sense of "pierce with a pointed stake" (as torture or capital punishment) first recorded 1610s. Related: Impaled; impaling.
impart (v.): early 15c., "to give a part of (one's possessions);" late 15c., "to share, take part in," from Old French empartir, impartir "assign, allot, allocate, share out" (14c.), from Late Latin impartire (also impertire) "to share in, divide with another; communicate," from assimilated form of in- "into, in" (see in- (2)) + partire "to divide, part" (see part (v.)). Meaning "communicate as knowledge or information" is from 1540s; the word was not originally restricted to immaterial things but now usually is only in reference to qualities. Related: Imparted; imparting; impartment.
impartial (adj.): "not partial, not favoring one over another," 1590s, from assimilated form of in- (1) "not, opposite of" + partial. First recorded use is in "Richard II." Related: Impartially.
impasse (n.): 1763, "blind alley, dead end," from French impasse "impassable road; blind alley; impasse" (18c.), from assimilated form of in- "not, opposite of" (see in- (1)) + passe "a passing," from passer "to pass" (see pass (v.)). Figurative use (c. 1840) is perhaps from its use in whist. Supposedly coined by Voltaire as a euphemism for cul de sac.
... dans l'impasse de St Thomas du Louvre; car j'appelle impasse, Messieurs, ce que vous appelez cul-de-sac: je trouve qu'une rue ne ressemble ni à un cul ni à un sac: je vous prie de vous servir du mot d'impasse, qui est noble, sonore, intelligible, nécessaire, au lieu de celui de cul, ... (etc.) [Voltaire, "A Messieurs Les Parisiens"]
impeach (v.): formerly also empeach, late 14c., "to impede, hinder, prevent," from Anglo-French empecher, Old French empeechier "to hinder, stop, impede; capture, trap, ensnare" (12c., Modern French empêcher), from Late Latin impedicare "to fetter, catch, entangle," from assimilated form of in- "into, in" (see in- (2)) + Latin pedica "a shackle, fetter," from pes (genitive pedis) "foot," from PIE root *ped- (1) "a foot" (see foot (n.)). Sense of "accuse a public officer of misconduct" first recorded 1560s, perhaps via Medieval Latin confusion of the word with Latin impetere "attack, accuse" (see impetus). Related: Impeached; impeaching.
impeccable (adj.): 1530s, "not capable of sin," from Middle French impeccable (15c.) or directly from Late Latin impeccabilis "not liable to sin," from assimilated form of in- "not, opposite of" (see in- (1)) + peccare "to sin" (see peccadillo). Meaning "faultless" is from 1610s. Related: Impeccably; impeccant; impeccancy.
impecunious (adj.): "lacking in money," 1590s, from assimilated form of in- "not, opposite of" (see in- (1)) + Latin pecuniosus "rich," from pecunia "money, property" (see pecuniary). Related: Impecuniously; impecuniosity.
impediment (n.): c. 1400, from Old French empedement or directly from Latin impedimentum "hindrance," from impedire "impede," literally "to shackle the feet," from assimilated form of in- "into, in" (see in- (2)) + pes (genitive pedis) "foot," from PIE root *ped- (1) "a foot" (see foot (n.)). Related: Impedimental.
impel (v.): early 15c., from Latin impellere "to push, strike against; set in motion, drive forward, urge on," from assimilated form of in- "into, in, on, upon" (see in- (2)) + pellere "to push, drive" (see pulse (n.1)). Related: Impelled; impelling.
impend (v.): "be about to happen" (usually of something unwanted), 1590s, from Latin impendere "to hang over;" figuratively "to be imminent, be near," from assimilated form of in- "into, in, on, upon" (see in- (2)) + pendere "hang" (see pendant). Literal sense in English is by 1780. Related: Impended; impending.
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).
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.
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.
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]
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.
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.
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.
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]
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.
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.
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.)).
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.