Online Etymology Dictionary

insue (v.): obsolete form of ensue. Related: Insued; insuing.
insula (n.): Latin, literally "an island" (also, in ancient Rome, "a block of buildings"); see isle. In anatomical use, the notion is "detached or standing out by itself."
insufficiency (n.): 1520s, from Old French insufficience and directly from Late Latin insufficientia "insufficience," noun of quality from insufficientem "insufficient" (see insufficient). Insufficience "deficiency" is from early 15c.
insubstantial (adj.): c. 1600, from Medieval Latin insubstantialis "not substantial," from in- "not" (see in- (1)) + Late Latin substantialis "having substance or reality, material," in Late Latin "pertaining to the substance or essence," from substantia "being, essence, material" (see substance). Related: Insubstantially.
instrumentation (n.): "composition and arrangement of music for instruments," 1836, from French instrumentation, from instrument "musical instrument" (see instrument (n.)); also see -ation.
instruct (v.): early 15c., "to tell, inform, impart knowledge or information," also "furnish with authoritative directions," from Latin instructus, past participle of instruere "arrange, prepare, set in order; inform, teach," literally "to build, erect," from in- "on" (see in- (2)) + struere "to pile, build" (see structure (n.)). Related: Instructed; instructing.
instinctive (adj.): 1640s, from Latin instinct-, past participle stem of instinguere "to incite, impel" (see instinct) + -ive. Related: Instinctively (1610s); instinctiveness. Coleridge uses instinctivity.
instantaneous (adj.): 1650s, from instant (n.) on model of spontaneous, etc. Related: Instantaneously (1640s); instantaneousness; instantaneity.
instable (adj.): c. 1400, from Latin instabilis "unsteady, not firm, inconstant, fickle," from in- "not, opposite of" (see in- (1)) + stabilis (see stable (adj.)). Now mostly replaced by unstable.
installation (n.): mid-15c., "action of installing," in reference to church offices or other positions, from Medieval Latin installationem (nominative installatio), noun of action from past participle stem of installare (see install). Of machinery, etc., "act of setting up, a placing in position for use," from 1882.
instantly (adv.): late 15c., "urgently, persistently," from instant (adj.) + -ly (2). Meaning "immediately, without any intervening time" is 1550s.
instigator (n.): 1590s, from Latin instigator "a stimulator," agent noun from instigare "urge on, incite" (see instigation). The classical Latin fem. form instigatrix is recorded in English from 1610s.
innervate (v.): "stimulate through the nerves," 1870, a back-formation from innervation "sending of a stimulus through the nerves" (1828), which is perhaps modeled on French innervation; see in- (2) "in" + nerve (n.) + -ate. Related: Innervated. Earlier in English the same word (but from the other in-) meant "to lose feeling or sensation" (1848), and, as an adjective, "without feeling" (1737). Innervation in psychology is from 1880, translated from German Innervationsgefühl.
inro: 1610s, from Japanese, from Chinese yin "seal" + lung "basket." The small ornamental baskets originally held seals, among other small items.
insipient (adj.): "foolish," mid-15c., from Latin insipientem (nominative insipiens) "unwise, foolish," from in- "not" (see in- (1)) + sapientem (see sapient). "Now mostly, or wholly, disused to avoid confusion with incipient" [OED].
insomniac: 1877 (adj.); 1879 (n.), from insomnia. Earlier was insomnious (1650s).
instantiate (v.): "represent by an instance," 1946, from instance (Latin instantia) + -ate. Related: Instantiated; instantiation.
instate (v.): also enstate, "to put someone in a certain state or condition," c. 1600, from in + state (n.1). Related: Instated; instating.
instillation (n.): 1540s, from Latin instillationem (nominative instillatio) "a dropping in," noun of action from past participle stem of instillare "put in by drops; to drop, trickle," from in- "in" (see in- (2)) + stilla "a drop" (see distill).
intra-ocular (adj.): also intraocular, 1826, from intra- + ocular.
intra-peritoneal (adj.): also intraperitoneal, "within the cavity of the peritoeum," 1835, from intra- "within" + peritoneal.
intra-personal (adj.): also intrapersonal, 1853, from intra- "within" + personal.
intra-psychic (adj.): also intrapsychic, 1902, from intra- "within" + psychic.
intricacy (n.): c. 1600, "state of being complex;" 1610s, "an intricate situation or condition," from intricate (adj.) + -cy. Related: Intricacies.
introductory (adj.): c. 1600, from Late Latin introductorius, from introduct-, past participle stem of Latin introducere "to lead in, bring in" (see introduction). Also used in English from c. 1400 as a noun meaning "introductory treatise or textbook."
introspect (v.): 1680s, "to look into" (transitive), from Latin introspectus, past participle of introspicere "look at, look into; examine, observe attentively," from intro- "inward" (see intro-) + specere "to look at" (see scope (n.1)). Meaning "look within, search one's feelings or thoughts" is from 1875, a back-formation from introspection. Related: Introspected; introspecting.
introversion (n.): 1650s, "action of turning inward" (of thought or contemplation), from Modern Latin introversionem, noun of action from past participle stem of *introvertere (see introvert (v.)). Psychological meaning "tendency to withdraw from the world" is from 1912.
intrude (v.): early 15c., in an ecclesiastical sense, "take possession of (a prebend) not rightfully one's own," a back-formation from intrusion, or else from Latin intrudere "to thrust in, force in," from in- "in" (see in- (2)) + trudere "to thrust, push," from PIE *treud- "to press, push, squeeze" (see threat).

From 1560s in a physical sense of "thrust in" (transitive or intransitive); meaning "enter unbidden and without welcome" is from 1570s; that of "thrust or bring in without necessity or right" is from 1580s. Related: Intruded; intruding.
intruder (n.): 1530s, agent noun from intrude. Originally legal. Fuller ("Pisgah-Sight of Palestine," 1650) has fem. form intrudress.
intumescent (adj.): "swelling up," 1796, from Latin intumescentem (nominative intumescens), present participle of intumescere "to swell up, rise, be elevated," of sounds, "grow louder," figuratively, "grow excited, become enraged," from in- "in" (see in- (2)) + tumescere "begin to swell, swell up" (see tumescence).
inturn (n.): 1590s, "turning in of the toes" (especially in dancing), from in + turn. In wrestling, "a lifting with the thigh" (c. 1600).
intuit (v.): 1776, "to tutor," from Latin intuit-, past participle stem of intueri "look at, consider," from in- "at, on" (see in- (2)) + tueri "to look at, watch over" (see tutor (n.)). Meaning "to perceive directly without reasoning, know by immediate perception" is from 1840 (De Quincey), in this sense perhaps a back-formation from intuition. Related: Intuited; intuiting.
intussusception (n.): "reception of one part within another," 1707, literally "a taking in," from Latin intus "within" (see ento-) + susceptionem (nominative susceptio) "a taking up, a taking in hand, undertaking," noun of action from past participle stem of suscipere "to take, catch, take up, lift up" (see susceptible).
inutile (adj.): late 15c., "unprofitable, useless," from French inutile (12c., inutele), from Latin inutilis "useless, unprofitable," from in- "not" (see in- (1)) + utilis "useful, beneficial, profitable," from uti "make use of, profit by, take advantage of" (see use (v.)). It appears to have fallen from use by 1700; modern use (from mid-19c.) is perhaps a reborrowing from French.
invader (n.): 1540s, agent noun from invade.
invaginate (v.): "put into a sheath," 1650s, from Latin in- "in" (see in- (2)) + verb from vagina "a sheath" (see vagina). Related: Invaginated; invagination.
invariant (adj.): 1795, from in- (1) "not" + variant (adj.). As a noun, in mathematics, from 1851. Related: Invariance.
inventive (adj.): early 15c., "skilled in invention," from Old French inventif (15c.), from Latin invent-, past participle stem of invenire "devise, discover, find" (see invention). Related: Inventively; inventiveness.
inventor (n.): c. 1500, "a discoverer, one who finds out" (now obsolete), from Latin inventor (fem. inventrix, source of French inventeur (15c.), Spanish inventor, Italian inventore) "contriver, author, discoverer, proposer, founder," agent noun from past participle stem of invenire "to come upon, find; find out; invent, discover, devise; ascertain; acquire, get earn," from in- "in, on" (see in- (2)) + venire "to come" (see venue). Meaning "one who contrives or produces a new thing or process" is from 1550s.
invent (v.): c. 1500, "to find, discover" (obsolete), a back-formation from invention or else from Latin inventus, past participle of invenire "to come upon; devise, discover." General sense of "make up, fabricate, concoct, devise" (a plot, excuse, etc.) is from 1530s, as is that of "produce by original thought, find out by original study or contrivance." Related: Invented; inventing.
invasive (adj.): mid-15c., from Middle French invasif (15c.) or directly from Medieval Latin invasivus "invasive," from invas-, past participle stem of invadere "go into; attack, invade," from in- "in" (see in- (2)) + vadere "go, walk" (see vamoose).
intubate (v.): 1610s, "to form into tubes," from in- (2) "in" + Latin tuba "tube" (see tuba) + -ate (2). Medical sense is from 1887. Related: Intubated. Intubation "act of inserting a tube" (into an orifice) is from 1885.
intrusive (adj.): c. 1400, "usurping," from Latin intrus-, past participle stem of intrudere (see intrusion) + -ive. Meaning "coming unbidden" is from 1640s. Geological sense "thrust in out of regular place" is from 1826. Related: Intrusively; intrusiveness.
inveteracy (n.): "long continuance," 1690s, from inveterate + -cy.
inviable (adj.): 1909, in biology, from in- (1) "not" + viable. Related: Inviability.
investment (n.): 1590s, "act of putting on vestments" (a sense now found in investiture); later "act of being invested with an office, right, endowment, etc." (1640s); and "surrounding and besieging" of a military target (1811); from invest + -ment.

Commercial sense of "an investing of money or capital" is from 1610s, originally in reference to the East India Company; general use is from 1740 in the sense of "conversion of money to property in hopes of profit," and by 1837 in the sense "amount of money invested." For evolution of the commercial senses, see invest.
investiture (n.): late 14c., "ceremony of clothing in the insignia of office," from Medieval Latin investitura "an investing," from past participle stem of Latin investire "to clothe" (see invest). Related: Investive.
investigator (n.): 1550s, a native agent-noun formation from investigate, or else from Latin investigator "he that searches into," agent noun from past participle stem of investigare "to trace out, search after" (see investigation). Related: Investigatorial.
investigative (adj.): 1803, from Latin investigat-, past participle stem of investigare (see investigation) + -ive. Journalism sense is from 1951.
inviolate (adj.): "unbroken, intact," early 15c., from Latin inviolatus "unhurt," from in- "not, opposite of" (see in- (1)) + violatus (see violation).