Online Etymology Dictionary

intellectual (adj.): late 14c., "grasped by the understanding" (rather than by the senses), from Old French intellectuel and directly from Latin intellectualis "relating to the understanding," from intellectus "discernment, understanding," from past participle stem of intelligere "to understand, discern" (see intelligence). Intellectual property attested from 1845. Other adjective formations included intellective (late 15c.), intellectile (1670s).
intellectual (n.): 1590s, "mind, intellect," from intellectual (adj.); sense of "an intellectual person" is from 1650s. Related: Intellectuals.
intellectualism (n.): 1829; see intellectual + -ism. Probably based on German Intellektualismus (said by Klein to have been coined 1803 by Friedrich Wilhelm Joseph von Schelling (1775-1854) from Late Latin intellectualis). In English, originally with reference to the doctrines of Leibnitz.
intellectuality (n.): mid-15c., "the part of the mind which understands; understanding, intellect;" from Old French intellectualité and directly from Late Latin intellectualitas, from Latin intellectualis (see intellectual).
intellectualization (n.): 1821, noun of action from intellectualize.
intellectualize (v.): 1819, from intellectual + -ize. Related: Intellectualized; intellectualizing.
intellectually (adv.): late 14c., from intellectual + -ly (2).
intelligence (n.): late 14c., "faculty of understanding," from Old French intelligence (12c.), from Latin intelligentia, intellegentia "understanding, power of discerning; art, skill, taste," from intelligentem (nominative intelligens) "discerning," present participle of intelligere "to understand, comprehend," from inter- "between" (see inter-) + legere "choose, pick out, read" (see lecture (n.)).

Meaning superior understanding, sagacity" is from early 15c. Sense of "information, news" first recorded mid-15c., especially "secret information from spies" (1580s). Intelligence quotient first recorded 1921 (see I.Q.).
intelligencer (n.): 1580s, "spy, informant," agent noun from intelligence. Meaning "bringer of news" is from 1630s; as a newspaper name from 1640s.
intelligent (adj.): c.1500, a back-formation from intelligence or else from Latin intelligentem (nominative intelligens), present participle of intelligere, earlier intellegere (see intelligence). Intelligent design, as a name for an alternative to atheistic cosmology and the theory of evolution, is from 1999. Related: Intelligently.
intelligentsia (n.): "the intellectual class collectively," 1905, from Russian intelligyentsia, from Latin intelligentia (see intelligence). Perhaps via Italian intelligenzia.
intelligibility (n.): 1670s, from intelligible + -ity.
intelligible (adj.): late 14c., "able to understand," from Latin intelligibilis, intellegibilis "that can understand, that can be understood," from intellegere "to understand" (see intelligence). In English, sense of "capable of being understood" first recorded c.1600. Related: Intelligibly.
intemperance (n.): early 15c., from Middle French intemperance (14c.), from Latin intemperantia "intemperateness, immoderation, excess," from in- "not, opposite of" (see in- (1)) + temperantia (see temperance). Originally of climate; meaning "lack of moderation" in English is from 1540s.
intemperate (adj.): "characterized by excessive indulgence in a passion or appetite," late 14c., from Latin intemperatus "untempered, inclement, immoderate," from in- "not, opposite of" (see in- (1)) + temperantia (see temperance). Related: Intemperately.
intend (v.): c.1300, "direct one's attention to," from Old French entendre, intendre "to direct one's attention" (in Modern French principally "to hear"), from Latin intendere "turn one's attention, strain," literally "stretch out, extend," from in- "toward" (see in- (2)) + tendere "to stretch" (see tenet). Sense of "have as a plan" (late 14c.) was present in Latin. A Germanic word for this was ettle, from Old Norse ætla "to think, conjecture, propose," from Proto-Germanic *ahta "consideration, attention" (cf. Old English eaht, German acht). Intended (n.) "one's intended husband or wife" is from 1767.
intendant (n.): "one who has charge of some business," 1650s, from French intendant (16c.), from Latin intendantem, present participle of intendere (see intend).
intense (adj.): c.1400, from Middle French intense (13c.), from Latin intensus "stretched, strained, tight," originally past participle of intendere "to stretch out, strain" (see intend); thus, literally, "high-strung." Related: Intensely.
intensification (n.): 1847, noun of action from intensify.
intensify (v.): 1817, from intense + -ify, first attested in Coleridge, in place of intend, which he said no longer was felt as connected with intense. Middle English used intensen (v.) "to increase (something), strengthen, intensify," early 15c. Related: Intensified; intensifying.
intension: c.1600, from Latin intensionem (nominative intensio) "a stretching, straining, effort," noun of action from past participle stem of intendere (see intend).
intensity: formed in English 1660s from intense + -ity. Earlier was intenseness (1610s). Sense of "extreme depth of feeling" first recorded 1830.
intensive (adj.): mid-15c., from French intensif (14c.), from Latin intens-, past participle stem of intendere (see intend). As a noun, 1813, from the adjective. Alternative intensitive is a malformation. Intensive care attested from 1958. Related: Intensively.
intent (n.): "purpose," early 13c., from Old French entente, from Latin intentus "a stretching out," in Late Latin "intention, attention," noun use of past participle of intendere "stretch out, lean toward, strain," literally "to stretch out" (see intend).
intent (adj.): "very attentive," late 14c., from Latin intentus "attentive, eager, waiting, strained," past participle of intendere "to strain, stretch" (see intend). Related: Intently.
intention (n.): mid-14c., from Old French entencion "stretching, intensity, will, thought" (12c.), from Latin intentionem (nominative intentio) "a stretching out, straining, exertion, effort; attention," noun of action from intendere "to turn one's attention," literally "to stretch out" (see intend).
intentional: 1520s, from Medieval Latin intentionalis, from intentionem (see intention). Intentional fallacy recorded from 1946. Related: Intentionality.
intentionally: "on purpose," 1660s; see intentional + -ly (2).
intentions: "one's purposes with regard to courtship and marriage," by 1796; see intention.
intentive: late 13c., from Old French ententif, intentif (12c.), from Late Latin intentivus, from intent-, past participle stem of intendere (see intend).
inter (v.): c.1300, from Old French enterer (11c.), from Medieval Latin interrare "put in the earth, bury," from Latin in- "in" (see in- (2)) + terra "earth" (see terrain). Related: Interred; interring.
inter alia: Latin, literally "amongst other things." (Latin for "among other persons" is inter alios), from inter "among, between" (see inter-).
inter-: Latin inter (prep., adv.) "among, between, betwixt, in the midst of," from PIE *enter "between, among" (cf. 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). Also in certain Latin phrases in English, such as inter alia "among other things." A living prefix in English from 15c. Spelled entre- in French, most words borrowed into English in that form were re-spelled 16c. to conform with Latin except entertain, enterprise.
inter-war: 1939, in reference to the period between the world wars, from inter- + war.
interact (v.): "to act on each other," 1805, from inter- + act (v.). Related: Interacted; interacting.
interaction (n.): 1812, from inter- + action.
interactive: 1832, from interact, probably on model of active. Related: Interactivity.
interbreed (v.): 1859, from inter- + breed. Related: Interbred; interbreeding.
intercalary (adj.): 1610s, from Latin intercalarius, from intercalare (see intercalate).
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 (see calendar). Related: Intercalated; intercalating.
intercalation: 1570s, from Latin intercalationem (nominative intercalatio) "insertion of an intercalary day," noun of action from past participle stem of intercalare (see intercalate).
intercede (v.): 1570s, a back-formation from intercession, or else from Latin intercedere "intervene, come between, be between," from inter- "between" (see inter-) + cedere "go" (see cede). Related: Interceded; interceding.
intercept (v.): c.1400, 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.
interception: early 15c., "action of intercepting" (the flow of a bodily fluid), from Latin interceptionem (nominative interceptio) "a seizing, taking away," noun of action from intercipere (see intercept). Specific football/rugby sense is attested by 1906. Meaning "action of closing in on and destroying an enemy aircraft, etc." is recorded from 1941.
interceptor: 1590s, from Latin interceptor, agent noun from intercipere (see intercept). As a type of fast fighter aircraft, from 1930.
intercession: early 15c., "act of interceding," from Latin intercessionem (nominative intercessio) "a going between," noun of action from past participle stem of intercedere (see intercede). The modern sense was not in classical Latin.
intercessor: late 15c., from Latin intercessor "one who intervenes," agent noun from intercedere (see intercede). Related: Intercessory.
interchange (v.): late 14c., from Old French entrechangier, from entre- (see inter-) + changier "to change" (see change (v.)). Related: Interchanged; interchanging.
interchange (n.): 1540s, "act of exchange, from Old French entrechange, from entrechangier (see interchange (v.)). In reference to a type of road junction, 1944.
interchangeable: late 14c. (implied in interchangeably), from inter- + changeable. Related: Interchangeability.