Online Etymology Dictionary

-ive: word-forming element making adjectives from verbs, meaning "pertaining to, tending to; doing, serving to do," in some cases from Old French -if, but usually directly from Latin adjectival suffix -ivus (source also of Italian and Spanish -ivo). In some words borrowed from French at an early date it has been reduced to -y (as in hasty, tardy).
-ical: compound adjectival word-forming element, usually interchangeable with -ic but sometimes with specialized sense (such as historic/historical, politic/political), Middle English, from Late Latin -icalis, from Latin -icus + -alis (see -al (1)). Probably it was needed because the forms in -ic often took on a noun sense (for example physic). Forms in -ical tend to be attested earlier in English than their twins in -ic.
interrogate (v.): late 15c., a back-formation from interrogation or else from Latin interrogatus, past participle of interrogare "to ask, question." The Old French word was interroger (14c.) which yielded English interroge (late 15c.), now obsolete. Related: Interrogated; interrogating.
gastro-intestinal (adj.): also gastrointestinal, 1821, from gastro- + intestinal.
inertial (adj.): "pertaining to inertia," 1737, from inertia + -al (1). Related: Inertially.
inevitability (n.): 1640s, from inevitable + -ity. Perhaps modeled on French inévitabilité.
inertness (n.): "inactivity; fact of being inert," 1660s, from inert + -ness.
inexpiable (adj.): 1560s, from Latin inexpiabilis "that cannot be atoned for," from in- "not" (see in- (1)) + expiabilis, from expiare "make amends for, purify, purge by sacrifice" (see expiation). The Inexpiable War was between Carthage and its Libyan mercenaries after the end of the First Punic War (241 B.C.E.). Related: Inexpiably.
inevitably (adv.): mid-15c., from inevitable + -ly (2).
inexactitude (n.): 1786, from in- (1) "not, opposite of" + exactitude. Perhaps modeled on French inexactitude (18c.).
inextinguishable (adj.): c. 1500, from in- (2) "not" + extinguishable. Earlier was inextinguible (early 15c.), from Old French inextinguible or directly from Latin inextinguibilis. Related: Inextinguishably; inextinguishability.
inexpugnable (adj.): late 15c., from Old French inexpugnable (14c.) or directly from Latin inexpugnabilis "not to be taken by assault, not to be rooted out, invincible," from in- "not" (see in- (1)) + expugnabilis "assailable," from expugnare (see expugn). Figurative sense, in reference to arguments, etc., is from 1530s.
infallibility (n.): "quality of being incapable of error," 1610s, from Medieval Latin infallibilitas, from infallibilis (see infallible).
infantilism (n.): 1894 in the psychological sense; see infantile + -ism. Earlier in a physiological sense, "retarded and imperfect physical development," perhaps from French infantilisme (1871).
infantryman (n.): 1837, from infantry + man (n.).
infeasibility (n.): 1650s, from infeasible + -ity.
Infanta (n.): "daughter of a king of Spain or Portugal," c. 1600, from Spanish and Portuguese infanta, fem. of infante "a youth; a prince of royal blood," from Latin infantem (see infant).
infibulate (v.): "to clasp, confine with a buckle, ring, clasp, or the like," especially of the sexual organs, to prevent copulation, 1620s, from Latin infibulatus, past participle of infibulare "to close with a clasp," from in- "on" (see in- (2)) + fibula "a clasp, pin" (see fibula). Related: Infibulated.
This operation was very generally practised in antiquity upon both young men and young women, but in later times chiefly upon the latter; and it is said to be still in use in some parts of the East. [Century Dictionary, 1902]
infinitely (adv.): early 15c., from infinite + -ly (2).
inflationary (adj.): 1916, from inflation + -ary.
inflexibility (n.): 1610s, from inflexible + -ity.
inflow (n.): 1839, from in (adj.) + flow (n.).
influence (v.): 1650s, from influence (n.). Related: Influenced; influencing.
info-: word-forming element abstracted from information.
informer (n.): late 14c., enfourmer "instructor, one who teaches or gives advice," from inform (Middle English enfourmen) and also from Old French enformeor. Meaning "one who communicates information" is mid-15c.; sense of "one who gives information against another" (especially in reference to law-breaking) is c. 1500.
informality (n.): 1590s, from informal + -ity.
infra (adv.): "under, below, further on," from Latin infra "below, under, beneath" (see infra-). A Latin word sometimes encountered in footnotes.
infrequency (n.): 1670s, fact of being infrequent," from Latin infrequentia "a small number, thinness, scantiness," noun of quality from infrequentem (nominative infrequens) "occurring seldom, unusual; not crowded, absent," from in- "not" (see in- (1)) + frequens (see frequent). Older in this sense is infrequence (1640s). Earlier infrequency was used in a now-obsolete sense of "state of being unfrequented" (c. 1600).
infrigidation (n.): early 15c., in medicine, "a making cold, cooling; a state of coolness," from Late Latin infrigidationem (nominative infrigidatio) "a cooling," noun of action from past participle stem of infrigidare "to make cold," from in- "in, into" (see in- (2)) + frigidare, from frigidus "cold" (see frigid). A verb infrigidate is attested from 1540s.
in-going (adj.): also ingoing, 1825, from in + going. Probably a modern formation unrelated to Middle English in-going (n.) "act of entering" (mid-14c.), from ingo "to go in, enter," from Old English ingan (past tense ineode), equivalent of German eingehen, Dutch ingaan.
ingestion (n.): "action of ingesting," 1610s, from Late Latin ingestionem (nominative ingestio) "a pouring in," noun of action from past participle stem of ingerere "pour in" (see ingest).
inharmonious (adj.): 1711, from in- (1) "not" + harmonious. Related: Inharmoniously.
inhumation (n.): "act of burying in the ground" (as opposed to cremation), 1630s, noun of action from inhume "to bury" (see inhume).
inimitability (n.): 1711, from inimitable + -ity. Perhaps from or modeled on French inimitabilité.
initiatory (adj.): 1610s, from initiate (v.) + -ory.
inlandish (adj.): 1650s, "produced at home, domestic, native," from inland in the "domestic, not foreign" sense + -ish. Also "characteristic of inland regions" (1849). Old English had inlendisc, inlende "native, indigenous."
inly (adv.): Old English inlice "internally, inwardly; sincerely, heartily;" see in + -ly (2).
ink (v.): "to mark or stain in ink," 1560s, from ink (n.). Meaning "to cover (a printing plate, etc.) with ink" is from 1727. Related: Inked; inks; inking.
inky (adj.): "as black as ink," 1590s, from ink (n.) + -y (2). Related: Inkily; inkiness.
injunctive: 1620s, from Latin iniunct-, past participle stem of iniungere "impose; attach to" (see injunction) + -ive. As a term in grammar, from 1910.
in-migration (n.): 1942, American English, in reference to movement within the same country (as distinguished from immigration), from in (prep.) + migration.
inmost (adj.): 16c. respelling of Middle English innemest, from Old English innemest "furthest within, remotest from the boundary;" see in + -most.
innkeeper (n.): 1540s, from inn + keeper.
innovator (n.): "an introducer of changes," 1590s, from Late Latin innovator, agent noun from innovare "to change" (see innovate).
inobservant (adj.): 1660s, from Late Latin inobservantem (nominative inobservans) "inattentive, negligent," from in- "not" (see in- (1)) + Latin observans (see observance). Related: Inobservance (1610s).
inquirer (n.): 1560s, "one who inquires, a seeker, an investigator," agent noun from inquire.
insemination (n.): 1650s, "action of sowing," noun of action from inseminate. Meaning "infusion of semen" is from 1854.
inscrutability (n.): 1650s, from inscrutable + -ity.
insatiability (n.): 1650s, from Late Latin insatiabilitas, from Latin insatiabilis "not to be satisfied" (see insatiable). Possibly via French insatiabilité (16c.).
inseparability (n.): 1620s, from Late Latin inseparabilitas "inseparableness," from Latin inseparabilis "that cannot be separated," from in- "not, opposite of" (see in- (1)) + separabilis, from separare (see separate (v.)).