Online Etymology Dictionary

implacability (n.): 1530s, from Late Latin implacabilitas, from Latin implacabilis "unappeasable" (see implacable).
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).
implausibility (n.): 1630s, from implausible + -ity.
implement (v.): "to complete, perform, carry into effect," 1707, originally chiefly in Scottish English, where the noun was a legal term meaning "fulfillment," from implement (n.). It spawned implementation, which is first recorded 1913. Related: Implemented; implementing.
implicitly (adv.): c. 1600, "by implication," from implicit + -ly (2). From 1640s as "unquestioningly."
import (n.): 1580s, "consequence, importance;" 1680s, "that which is imported;" both from import (v.).
importation (n.): c. 1600; see import (v.) + -ation.
imposing (adj.): "impressive in appearance or manner," 1786, present participle adjective from impose (v.). Related: Imposingly.
impoundment (n.): 1660s; see impound + -ment. Earlier in the same sense were impoundage (1610s), impounding (1550s).
impregnation (n.): late 14c., "making or becoming pregnant," from Old French impregnacion or directly from Late Latin impregnationem (nominative impregnatio), noun of action from past participle stem of impraegnare "to impregnate" (see impregnate).
impressionistic (adj.): "of or pertaining to impressionists or their work," 1883; see impressionist + -ic.
improbability (n.): 1590s, "fact or quality of being improbable;" see improbable + -ity. Meaning "an instance of something improbable" is from 1610s.
improvidence (n.): "lack of foresight, rashness," mid-15c., from Late Latin improvidentia, from assimilated form of in- "not" (see in- (1)) + Latin providentia "foresight, precaution" (see providence).
imprisonment (n.): late 14c., from Anglo-French emprisonement (13c.), Old French emprisonnement "capture, imprisonment" (13c.), from emprisoner (see imprison).
improvisational (adj.): 1879; see improvisation + -al (1). Earlier adjectives were improvisatorial (1819), improvisatory (1806).
impulsively (adv.): 1751, from impulsive + -ly (2).
impulsiveness (n.): 1650s, from impulsive + -ness.
impulsivity (n.): 1891; see impulsive + -ity.
imputable (adj.): 1620s, from Medieval Latin imputabilis, from Latin imputare "to charge, ascribe" (see impute). Related: Imputability.
impress (n.): "act of impressing" (1590s), also "characteristic mark" (1580s), from impress (v.1). From 1620s as "badge worn by nobility or their retainers," from Italian impresa; earlier in English in this sense as impreso, imprese (1580s).
ileo-: word-forming element, from comb. form of ileum (q.v.).
ill (v.): early 13c., "do evil to," from ill (adj.). Meaning "speak disparagingly" is from 1520s. Related: Illed; illing.
ill (adv.): c. 1200, "wickedly; with hostility," from ill (adj.). Meaning "not well, poorly" also is from c. 1200. It generally has not shifted to the realm of physical sickness, as the adjective has done. Ill-fated recorded from 1710; ill-informed from 1824; ill-tempered from c. 1600; ill-starred from c. 1600. Generally contrasted with well, hence the useful, but now obsolete or obscure illcome (1570s), illfare (c. 1300), and illth.
illing (n.): "evil-doing, malevolent treatment," early 13c., verbal noun from ill (v.).
illocution (n.): 1955, from assimilated form of in- (1) "not, opposite of" + locution.
illy (adv.): "in an ill manner," 1540s, from ill (adj.) + -ly (2). Correctly formed but seldom used; simple ill generally serving as the adverb.
immemorable (adj.): "not memorable," 1550s, from Latin immemorabilis "not worth mentioning; silent," from assimilated form of in- "not" (see in- (1)) + memorabilis (see memorable). In English it occasionally has been used to mean "old beyond memory," but that sense is best left to immemorial.
immobilization (n.): 1846, noun of action from immobilize.
immitigable (adj.): 1570s, from Latin immitigabilis, from assimilated form of in- "not" (see in- (1)) + mitigabilis, from past participle stem of mitigare "make mild or gentle" (see mitigate). Related: Immitigably.
immovability (n.): late 14c., immoevablete, "quality of being unchanging," from immovable + -ity.
immortalization (n.): c. 1600, noun of action or state from immortalize.
impassivity (n.): 1789, from impassive + -ity. Earlier in the same sense was impassiveness (1640s).
impeachable (adj.): c. 1500, from impeach + -able. Related: impeachably; impeachability.
imperceptibility (n.): 1670s, from imperceptible + -ity.
impersonality (n.): 1769, from impersonal + -ity.
impertinently (adv.): mid-15c., "not to the point, irrelevantly," from impertinent + -ly (2). Meaning "intrusively, presumptuously" is from 1640s.
impoverishment (n.): 1550s, from Anglo-French empoverissement, from empoverir; see impoverish + -ment.
inferiority (n.): "state of being inferior," 1590s, probably from Medieval Latin *inferioritas; see inferior + -ity. Inferiority complex first attested 1919.
The surrender of life is nothing to sinking down into acknowledgment of inferiority. [John C. Calhoun]
be-in (n.): "a public gathering of hippies" [OED], 1967, from be + in.
cast iron (n.): 1660s, from cast "made by melting and being left to harden in a mold" (1530s), past participle adjective from cast (v.) in its sense "to throw something (in a particular way)," c. 1300, especially "form metal into a shape by pouring it molten" (1510s). From 1690s as an adjective, cast-iron.
-ics: in the names of sciences or disciplines (acoustics, aerobics, economics, etc.), a 16c. revival of the classical custom of using the neuter plural of adjectives with Greek -ikos "pertaining to" (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their English names before c. 1500, however, tend to be singular in form (arithmetic, logic, magic, music, rhetoric). The grammatical number of words in -ics (mathematics is/mathematics are) is a confused question.
coitus interruptus (n.): 1900, first attested in Havelock Ellis.
idealism (n.): 1796 in the abstract metaphysical sense "belief that reality is made up only of ideas," from ideal (adj.) + -ism. Probably formed on model of French idéalisme. Meaning "tendency to represent things in an ideal form" is from 1829. Meaning "pursuit of the ideal, a striving after the perfect state" (of truth, purity, justice, etc.).

In the philosophical sense the Germans have refined it into absolute (Hegel), subjective (Fichte), objective (von Schelling), and transcendental (Kant).
idealist (n.): "one who represents things in an ideal form," 1829, from ideal + -ist. Earlier (1796) in a philosophical sense "one who believes reality consists only in (Platonic) ideals" (see idealism).
It seems even incredible, that any Idealist in any age could forget himself so far as to run his head against a post, merely because he found in his system, that no external world does exist, and that therefore nothing could be without to hurt him. [F.A. Nitsch, "A General and Introductory View of Professor Kant's Principles," 1796]
Earlier still, "one who holds doctrines of philosophical idealism" (1701).
ideally (adv.): "in the best conceivable situation," 1840, from ideal + -ly (2). Earlier "in an archetype" (1640s); "in idea or imagination" (1590s).
I.D.: also ID (but pronounced as separate letters), short for identification, attested from 1955.
idiomatic (adj.): 1712, "peculiar to a certain language," from Latin idiomaticus, from Greek idiomatikos "peculiar, characteristic;" from idios "one's own" (see idiom) + matos "thinking, animated" (see automaton). Meaning "marked by use of idioms" is from 1839.
idolize (v.): 1590s, "to admire excessively," from idol + -ize. Literal sense "worship as an idol" is from 1660s. Related: Idolized; idolizing.
iffy (adj.): 1937, American English, from if + -y (2). Originally associated with President Franklin D. Roosevelt.
illuminate (v.): c. 1500, "to light up, shine on," a back-formation from illumination or else from Latin illuminatus, past participle of illuminare "light up, make light, illuminate." Earlier was enlumyen (late 14c.) "decorate written material by hand with gold, silver, or bright colors," from Old French enluminer, from Late Latin inluminare; also illumine (late 14c.). Related: Illuminated; illuminating; illuminable.