Online Etymology Dictionary

Inigo: masc. proper name, from Spanish Iñigo, probably from Latin Ignatius.
inimical (adj.): 1640s, from Late Latin inimicalis "hostile," from Latin inimicus "unfriendly; an enemy" (see enemy).
Inimical expresses both feeling and action, generally in private affairs. Hostile also expresses both feeling and action, but applies especially to public affairs: where it applies to private matters, it expresses either strong or conspicuous action or feeling, or both, or all. [Century Dictionary, 1902]
inimitable (adj.): late 15c., from Latin inimitabilis "that cannot be imitated," from in- "not, opposite of" (see in- (1)) + imitabilis "imitable" (see imitable). Related: Inimitably.
iniquity (n.): c. 1300, "hostility, malevolence; a hostile action," from Old French iniquité, iniquiteit "wickedness; unfavorable situation" (12c.), from Latin iniquitatem (nominative iniquitas) "unequalness, unevenness," figuratively "unfavorableness, unfairness, injustice," noun of quality from iniquus "unjust, unequal; slanting, steep," from in- "not" (see in- (1)) + aequus "just, equal" (see equal (adj.)).

For the vowel change in the Latin compound, see acquisition. Meaning "evil, wickedness" is from late 14c. Old Iniquity (1610s) was a comic or buffoonish character in old morality plays, representing vice.
initial (adj.): 1520s, "of or pertaining to a beginning," from Middle French initial or directly from Latin initialis "initial, incipient, of the beginning," from initium "a beginning, a commencement; an entrance, a going in," noun use of neuter past participle of inire "to go into, enter upon, begin," from in- "into, in" (see in- (2)) + ire "to go" (see ion). Related: Initially.
initiation (n.): 1580s, from Middle French initiation or directly from Latin initiationem (nominative initiatio) "participation in secret rites," noun of action from past participle stem of initiare "originate, initiate," from initium "a beginning" (see initial (adj.)).
initiative (n.): "power of initiating," 1775, from French initiative (16c.), from Latin initiatus, past participle of initiare "to begin," from initium "a beginning" (see initial (adj.)). From 1793 as "disposition to take the lead." Phrase take the initiative recorded by 1815.
inject (v.): c. 1600, in medicine, from specialized sense of Latin iniectus "a casting on, a throwing over," past participle of inicere "to throw in or on; insert, bring into," from in- "in" (see in- (2)) + -icere, comb. form of iacere "to throw" (see jet (v.)). Related: Injectable; injected; injecting.
in-joke (n.): 1964, from in (adj.) + joke (n.).
injudicious (adj.): 1640s, "incapable of judging aright, wanting good judgement," from in- (1) "not, opposite of" + judicious. Meaning "ill-advised" is from 1711. In the older sense the earlier English word was injudicial (c. 1600). Related: Injudiciously; injudiciousness.
Injun (n.): 1812 (from 1683 as Ingin), a spelling representing the early American English colloquial pronunciation of Indian (q.v.). Honest Injun as an asseveration of truthfuless is first recorded 1868, from the notion of assurance extracted from Indians of their lack of duplicity in a particular situation.
"Honest Injun?" inquired Mr. Wilder, using a Western phrase equivalent to demanding of the narrator of a story whether he is strictly adhering to the truth. ["The Genial Showman," London, 1870]
The noun phrase honest Indian itself is attested from 1676 in Massachusetts.
intense (adj.): early 15c., of situations or qualities, "great, extreme," from Old French intense (13c.), from Latin intensus "stretched, strained, high-strung, tight," originally past participle of intendere in its literal sense of "stretch out, strain" (see intend). From 1630s of persons, "high-strung." Related: Intensely; intenseness.
intent (n.): "purpose," early 13c., from Old French entent, entente "goal, end, aim, purpose; attention, application," and directly from Latin intentus "a stretching out," in Late Latin "intention, purpose," noun use of past participle of intendere "stretch out, lean toward, strain," literally "to stretch out" (see intend). In law, "state of mind with respect to intelligent volition" (17c.).
intent (adj.): late 14c., "very attentive, eager," from Latin intentus "attentive, eager, waiting, strained," past participle of intendere "to strain, stretch" (see intend). Sense of "having the mind fixed (upon something)" is from c. 1600. Related: Intently.
inter (v.): "bury in the earth or a grave," c. 1300, formerly also enter, from Old French enterer (11c.), from Medieval Latin interrare "put in the earth, bury," from in- "in" (see in- (2)) + Latin terra "earth" (see terrain). Related: Interred; interring.
inter-: word-forming element used freely in English, "between, among, during," from Latin inter (prep., adv.) "among, between, betwixt, in the midst of" (also used extensively as a prefix), from PIE *enter "between, among" (source also of Sanskrit antar, Old Persian antar "among, between," Greek entera (plural) "intestines," Old Irish eter, Old Welsh ithr "among, between," Gothic undar, Old English under "under"), a comparative of *en "in" (see in (adv.)).

A living prefix in English from 15c. and used with Germanic as well as Latinate words. Spelled entre- in French; most words borrowed into English in that form were re-spelled 16c. to conform with Latin except entertain, enterprise. In Latin, spelling shifted to intel- before -l-, hence intelligence, etc.
interaction (n.): 1812, from inter- + action.
intercalate (v.): "to insert a day into the calendar," 1610s, from Latin intercalatus, past participle of intercalare "to proclaim the insertion of an intercalary day," from inter- "between" (see inter-) + calare "to call" (an intercalary day; see calendar). Related: Intercalated; intercalating.

A necessary process in the Roman calendar to balance the solar and lunar aspects of it. Intercalation was done after Feb. 23 or 24 (the terminalia), every two or four years. Twenty-seven days were intercalated, making a full intercalary month (which included the last four or five days of Februarius), known as mensis intercalaris (and also known, according to Plutarch, as Mercedonius). No one now knows why the intercalation was done in the middle of February rather than after its end, unless it was because the important festivals at the end of that month (Regifugium and Equirra) were closely associated with holidays in early March. After Caesar's reform (46 B.C.E.) the only intercalary day is Feb. 29 every four years.
intercede (v.): 1570s, "to come between in space or time" (obsolete); c. 1600, "to interpose on behalf of another," a back-formation from intercession, or else from Latin intercedere "intervene, come between, be between" (in Medieval Latin "to interpose on someone's behalf"), from inter- "between" (see inter-) + cedere "go" (see cede). Related: Interceded; interceding.
intercept (v.): c. 1400, "to cut off" (a line), "prevent" (the spread of a disease), from Latin interceptus, past participle of intercipere "take or seize between, to seize in passing," from inter- "between" (see inter-) + -cipere, comb. form of capere "to take, catch" (see capable). Related: Intercepted; intercepting.
intercession (n.): early 15c., "act of interceding;" c. 1500, "intercessory prayer, a pleading on behalf of oneself or another," from Latin intercessionem (nominative intercessio) "a going between, coming between, mediation," noun of action from past participle stem of intercedere "intervene, come between, be between" (in Medieval Latin "to interpose on someone's behalf;" see intercede). The sense "pleading on behalf of another" developed in Christianity.
interchange (v.): late 14c., enterchaungen, "to give and receive reciprocally; to alternate, put each in place of the other" (trans.), also "change reciprocally" (intrans.), from Old French entrechangier "interchange, exchange," from entre- "between" (see inter-) + changier "to change" (see change (v.)). Related: Interchanged; interchanging.
intercom (n.): "radio or telephone intercommunication system," 1937, colloquial shortening of intercommunication, which is attested from 1911 in reference to systems of linked telephones.
intercourse (n.): mid-15c., "communication to and fro," ("In early use exclusively with reference to trade" [OED]), from Old French entrecors "exchange, commerce, communication" (12c., Modern French entrecours), from Late Latin intercursus "a running between, intervention," in Medieval Latin "intercommunication," from intercursus, past participle of intercurrere "to run between, intervene, mediate," from Latin inter- "between" (see inter-) + currere "to run" (see current (adj.)).

Sense of "frequent and habitual meeting and contact, social communication between persons" is from 1540s. Meaning "mental or spiritual exchange or intercommunication" is from 1560s. Meaning "sexual relations" (1798) probably is a shortening of euphemistic sexual intercourse (1771) with intercourse in its sense "social contact and relations."
interdict (v.): c. 1300, enterditen, "to place under ban of the Church, excommunicate," from Old French entredit (Modern French interdit), past participle of entredire "forbid by decree, excommunicate," from Latin interdicere "interpose by speech, prohibit, forbid," from inter- "between" (see inter-) + dicere "to speak, to say" (see diction). General sense "forbid, prohibit" in English is from early 15c. Related: Interdicted; interdicting; interdictory.
interest (n.): mid-15c., "legal claim or right; a concern; a benefit, advantage, a being concerned or affected (advantageously)," from Old French interest "damage, loss, harm" (Modern French intérêt), from noun use of Latin interest "it is of importance, it makes a difference," third person singular present of interresse "to concern, make a difference, be of importance," literally "to be between," from inter- "between" (see inter-) + esse "to be" (see essence). The sense development to "profit, advantage" in French and English is not entirely clear.

The earlier Middle English word was interesse (late 14c.), from Anglo-French interesse "what one has a legal concern in," from Medieval Latin interesse "compensation for loss," noun use of Latin interresse (compare German Interesse, from the same Medieval Latin source).

Financial sense of "money paid for the use of money lent" (1520s) earlier was distinguished from usury (illegal under Church law) by being in reference to "compensation due from a defaulting debtor." Sense of "personal or selfish consideration" is from 1620s. Meaning "business in which several people are interested" is from 1670s. Meaning "curiosity, feeling that something concerns one, appreciative or sympathetic regard" is first attested 1771. Interest group is attested from 1907; interest rate by 1868.
interface (n.): 1874, "a plane surface regarded as the common boundary of two bodies," from inter- + face (n.). Modern use is perhaps a c. 1960 re-coinage; McLuhan used it in the sense "place of interaction between two systems" (1962) and the computer sense "apparatus to connect two devices" is from 1964. As a verb from 1967. Related: Interfaced; interfacing.
interfere (v.): formerly also enterfere, mid-15c., "to strike against," from Middle French enterferir "exchange blows, strike each other," from entre- "between" (see entre-) + ferir "to strike," from Latin ferire "to knock, strike," related to Latin forare "to bore, pierce" (see bore (v.), and compare punch (v.), which has both the senses "to hit" and "to make a hole in").

Figurative sense of "to meddle with, oppose unrightfully" is from 1630s. Related: Interfered; interfering. Modern French interférer is from English.
interferon (n.): animal protein, 1957, coined in English from interfere + subatomic particle suffix -on; so called because it "interferes" with the reduplication of viruses.
interim (n.): "time intervening," 1560s, from Latin interim (adv.) "in the meantime, meanwhile," originally "in the midst of that," from inter- "between" (see inter-) + im, ancient adverb from stem of pronoun is "this, that." As an adjective from c. 1600.
interior (adj.): late 15c., from Latin interior "inner, interior, middle," comparative adjective of inter "within" (see inter-). Specific meaning "away from the coast, of the interior parts of a country" is from 1777. Interior decoration first attested 1769; interior decorator is from 1830. Interior design from 1927.
interjection (n.): early 15c., "an interjected or exclamatory word," from Middle French interjection (Old French interjeccion, 13c.), from Latin interiectionem (nominative interiectio) "a throwing or placing between," also in grammar and rhetoric, noun of action from past participle stem of intericere "to throw between, set between," from inter- "between" (see inter-) + -icere, comb. form of iacere "to throw" (see jet (v.)). Related: Interjectional.
interlard (v.): early 15c., "to mix with alternate layers of fat" (before cooking), from Old French entrelarder (12c.), from entre- "between" (see inter-) + larder "to lard," from Old French lard "bacon fat" (see lard (n.)). Figurative sense of "diversify with something intermixed" first recorded 1560s. Related: Interlarded; interlarding; interlardment.
interlocutor (n.): 1510s, "one who speaks in a dialogue or conversation," agent noun from Latin interlocut-, past participle stem of interloqui "speak between; interrupt," from inter- "between" (see inter-) + loqui "to speak" (see locution).

In minstrel shows, the name of a straight-man character (1870) who was the questioner of the end men. Related: Interlocutory. Fem. forms include interlocutress (1858), interlocutrix (1846), interlocutrice (1848).
interloper (n.): 1590s, enterloper, "unauthorized trader trespassing on privileges of chartered companies," probably a hybrid from inter- "between" + -loper (from landloper "vagabond, adventurer," also, according to Johnson, "a term of reproach used by seamen of those who pass their lives on shore"); perhaps from a dialectal form of leap, or from Middle Dutch loper "runner, rover," from lopen "to run," from Proto-Germanic *hlaupan "to leap" (see leap (v.)).

OED says Dutch enterlooper "a coasting vessel; a smuggler" is later than the English word and said by Dutch sources to be from English. General sense of "self-interested intruder" is from 1630s.
interlude (n.): formerly also enterlude, c. 1300, from Old French entrelude and directly from Medieval Latin interludium "an interlude," from Latin inter "between" (see inter-) + ludus "a play" (see ludicrous). Originally the term for farcical episodes drawn from real life introduced between acts of long mystery plays. In 17c.-18c. it meant "popular stage play;" transferred (non-dramatic) sense of "interval in the course of some action" is from 1751. Related: Interludial.
intermarry (v.): 1570s, "to marry one another," from inter- + marry (v.). Meaning "to marry across families, castes, tribes, etc." is from 1610s. Related: Intermarried; intermarrying.
intermediate (adj.): "being or occurring between" (two things), early 15c., from Medieval Latin intermediatus "lying between," from Latin intermedius "that which is between," from inter- "between" (see inter-) + medius "in the middle" (see medial (adj.)).
intermezzo (n.): 1782, from Italian intermezzo "short dramatic performance (usually light and pleasing) between the acts of a play or opera," literally "that which is between," from Latin intermedius (see intermediate (adj.)).
interminable (adj.): late 14c., from Old French interminable (14c.) or directly from Late Latin interminabilis "endless," from in- "not" (see in- (1)) + terminabilis, from terminare "to limit, set bounds, end" (see terminus (adj.)). Related: Interminably.
intermission (n.): early 15c., "fact of intermitting, temporary pause," from Latin intermissionem (nominative intermissio) "a breaking off, discontinuance, interruption," noun of action from past participle stem of intermittere "to leave off, leave an interval," from inter- "between" (see inter-) + mittere "let go, send" (see mission). Meaning "lapse of time between events" is from 1560s; specifically of performances (originally plays, later movies, etc.) from 1854.
Intermission is used in U.S. for what we call an interval (in a musical or dramatic performance). Under the influence of LOVE OF THE LONG WORD, it is beginning to infiltrate here and should be repelled; our own word does very well. [H.W. Fowler, "Modern English Usage," 1926]
intermittent (adj.): c. 1600, from Latin intermittentem (nominative intermittens), present participle of intermittere "to leave off, cease, pause" (see intermission). Related: Intermittently.
intermural (adj.): 1650s, from Latin intermuralis "situated between walls," from inter- "between" (see inter-) + murus (genitive muralis) "wall" (see mural).
intern (v.1): 1866, "to confine within set limits," from French interner "send to the interior, confine," from Middle French interne "inner, internal" (14c.), from Latin internus "within, internal" (see internal; also compare intern (n.)).
intern (n.): "one working under supervision as part of professional training," originally "assistant resident physician, doctor in training in a hospital," 1879, American English, from French interne "assistant doctor," noun use of interne "internal," from Latin internus "within, inward" (see intern (v.)). Extended to other professions (originally teaching) from 1963 in reference to one under training and acquiring practical experience.
internal (adj.): early 15c., "extending toward the interior," from Medieval Latin internalis, from Latin internus "within, inward, internal," figuratively "domestic," expanded from pre-Latin *interos, *interus "on the inside, inward," from PIE *en-ter- (source also of Old Church Slavonic anter, Sanskrit antar "within, between," Old High German unter "between," and the "down" sense of Old English under); suffixed (comparative) form of *en "in" (see in (adv.)).

Meaning "situated within" is from 1590s. Meaning "of or pertaining to the domestic affairs of a country (as in internal revenue) is from 1795; the notion is "pertaining to the subject itself; independent of others." Internal-combustion in reference to an engine in which fuel is burned inside it, is from 1884. Related: Internally.
internecine (adj.): 1660s, "deadly, destructive," from Latin internecinus "very deadly, murderous, destructive," from internecare "kill or destroy," from inter (see inter-) + necare "kill" (see noxious).

Considered by OED as misinterpreted in Johnson's Dictionary [1755], which defined it as "endeavouring mutual destruction," but a notion of "mutually destructive" has been imported into the word in English because in English inter- usually conveys the idea of "mutual." The Latin prefix is said to have had here only an intensive sense; "the Latin word meant merely of or to extermination ... without implying that of both parties" [Fowler].
Internet (n.): 1984, "the linked computer networks of the U.S. Defense Department," shortened from internetwork, inter-network, which was used from 1972 in reference to (then-hypothetical) networks involving many separate computers. From inter- "between" + network (n.).
interpersonal (adj.): 1911 (OED finds an isolated use from 1842), from inter- "between" + person (n.) + -al. Introduced in psychology (1938) by H.S. Sullivan (1892-1949) to describe behavior between people in an encounter. Related: Interpersonally.
interplanetary (adj.): 1690s, "existing between planets," from inter- "between" + planet + -ary. In reference to travel between planets, attested from 1897. Boyle and Locke both used intermundane in the same sense; the Roman Epicureans had intermundia (neuter plural) for "spaces between the worlds" (translating Greek metakosmia).