Online Etymology Dictionary

ironing (n.): "act of pressing and smoothing clothes with a heated flat-iron," 1725, verbal noun from iron (v.). Ironing-board attested from 1762.
Irishman (n.): c. 1200, from Irish (adj.) + man (n.).
irremovable (adj.): "not capable of or subject to removal," 1590s, from assimilated form of in- (1) "not, opposite of" + removable. Related: Irremovably; irremovability.
irreligion (n.): 1590s, from French irréligion (16c.) or directly from Late Latin irreligionem (nominative irreligio) "irreligion, impiety," from assimiliated form of in- "not" (see in- (1)) + religio (see religion).
ironwork (n.): also iron-work, "objects made of iron," early 15c., from iron (n.) + work (n.). Related: Iron-worker (15c.). Iron works "iron foundry" is from 1580s.
irrationality (n.): 1560s, originally in the mathematical sense, from irrational + -ity. Meaning "unreasonableness, absurdity" is from 1640s.
izzard (n.): old name for "Z," 1738, a variant of zed. The guess that it representes S-hard wants evidence.
inadvisability (n.): 1839, from inadvisable + -ity.
irresolution (n.): 1590s, from assimilated form of in- (1) "not, opposite of" + resolution. Perhaps from or based on French irrésolution (16c.).
irresolvable (adj.): 1650s, "insoluble," from assimilated form of in- (1) "not, opposite of" + resolvable. Meaning "that cannot be resolved into parts" is from 1785. Related: Irresolvably.
irresponsibility (n.): 1767; see irresponsible + -ity.
irrigable (adj.): 1813, from Latin stem of irrigate (v.) + -able.
irritability (n.): 1755, from irritable + -ity or else from Latin irribilitas.
Islamic (adj.): "pertaining to Islam," 1791, from Islam + -ic.
isocracy (n.): "equal power," 1650s, from Greek isokratia "equality of political rights," from isokrates "of equal power, possessing equal rights with (others)," from isos "equal" (see iso-) + -kratia "power, rule, authority" (see -cracy). Related: Isocratic.
isocephalic (adj.): "having the heads of the principal figures at about the same level," 1893, from Greek isokephalos "like-headed," from isos "equal" (see iso-) + kephale "head" (see cephalo-). The rule or principle of ancient Greek art that makes figures standing and on horseback have heads carved on nearly the same level.
Isold: fem. proper name, French Isolt, Iseut, of Germanic origin, literally "ice-rule," from *is "ice" (see ice (n.)) + *waltan (see wield).
isometrics (n.): as a type of exercise, 1962, American English, from isometric; also see -ics.
isothermal (adj.): 1816, literally "of equal heat," from French isotherme (see isotherm) + -al (1). As a noun, "isothermal line," from 1849.
Israelite (n.): mid-14c., "a Jew; one of the people of ancient Israel, a descendant of Israel or Jacob," from Latin israelita, from Greek Israelites, from Israel (see Israel). The Middle English adjective was Israelish (Old English Israelisc), sometimes Israelitish (Coverdale, 1530s); Israelitic (c. 1600, from Late Latin Israeliticus).
isthmian (adj.): c. 1600, from Latin isthmius, from Greek isthmios "pertaining to the isthmus (of Corinth)," from isthmos (see isthmus). Originally in reference to the Isthmian Games, held at Corinth in honor of Poseidon.
issue (v.): mid-14c., of water, etc., "to flow out;" of persons, "come or go (out of a place), sally forth," from issue (n.) or else from Old French issu, past participle of issir. Transitive sense of "to send out" is from mid-15c.; specific sense of "to send out authoritatively" is from c. 1600. Meaning "supply (someone with something)" is from 1925. Related: Issued; issuing.
itch (v.): Middle English icchen, from Old English giccan "to itch," from West Germanic *jukkjan (source also of Middle Dutch jöken "to itch," Old High German jucchen, German jucken). Figurative sense of "feel a provoking desire to do or get something" is from early 13c. Related: Itched; itching.
itty (adj.): 1798, in a letter of Jane Austen, baby-talk form of little (adj.). Compare itsy-bitsy.
-ion: word-forming element attached to verbs, making nouns of state, condition, or action, from French -ion or directly from Latin -ionem (nominative -io, genitive -ionis), common suffix forming abstract nouns from verbs.
institute (n.): 1510s, "purpose, design," from Latin institutum "an ordinance; a purpose; a custom; precedents; principal components," literally "thing set up," noun use of neuter past participle of instituere "to set up, put in place; arrange; found, establish" (see institute (v.)).

From 1540s in English as "an established law." The sense of "an organization or society devoted to some specific work," especially literary or scientific, is from 1828, from French use in Institut national des Sciences et des Arts (established 1795); Dutch instituut, German Institut also are from French. The specialized (mostly U.S.) sense "travelling academy for teachers in a district" is from 1839.
A "Teachers' Institute" is a meeting composed of teachers of Common Schools, assembled for the purpose of improvement in the studies they are to teach, and in the principles by which they are to govern. It is the design of a Teachers' Institute to bring together those who are actually engaged in teaching Common Schools, or who propose to become so, in order that they may be formed into classes and that these classes, under able instructers, may be exercised, questioned and drilled, in the same manner that the classes of a good Common School are exercised, questioned and drilled. [Horace Mann, secretary's report to the Boston Board of Education, Sept. 1, 1845]
intentionally (adv.): "on purpose," 1660s; see intentional + -ly (2). Middle English had the phrase of entencioun (1420) "on purpose, intentionally."
in extremis: "at the point of death," 16c., Latin, literally "in the farthest reaches," from ablative plural of extremus "extreme" (see extreme (adj.)).
-in (2): word-forming element in chemistry, usually indicating a neutral substance, antibiotic, vitamin, or hormone; a modification and specialized use of -ine (2).
inartistic (adj.): "not conformable to the rules or principles of art" [Century Dictionary], 1836, from in- (1) "not" + artistic. Inartistical is attested from 1849. Related: Inartistically.
incarnate (v.): "clothe or embody in flesh," 1530s, a back-formation from incarnation, or else from Late Latin incarnatus "made flesh," past participle of incarnare "to make flesh; be made flesh." Meaning "make or form flesh" (as in healing a wound) is from 1670s. Related: Incarnated; incarnating.
incendiarism (n.): 1670s; see incendiary + -ism. Originally figurative; the literal sense of "malicious burning" is attested from 1755.
incense (v.2): "to offer incense, perfume with incense, fumigate (something) with incense," late 13c., encensen, incensen, from incense (n.) or from Old French encenser (11c.), or directly from Medieval Latin incensare.
incessancy (n.): 1610s, from incessant + -cy.
incineration (n.): "act of burning to ashes," 1520s, from Middle French incinération (14c.), from Medieval Latin incinerationem (nominative incineratio), noun of action from past participle stem of incinerare "reduce to ashes" (see incinerate).
incitive (adj.): "inciting, instigating," 1725; see incite + -ive. Other adjectives that have been used are incitative (c. 1500), incitatory (c. 1600), incitory (1941).
inclemency (n.): 1550s, from Middle French inclémence and directly from Latin inclementia "rigor, harshness, roughness," from inclemens "harsh, unmerciful" (see inclement).
incogitable (adj.): "unthinkable, inconceivable," 1520s, from Late Latin incogitabilis "unthinking; unthinkable," from in- "not" (see in- (1)) + cogitabilis "thinkable, conceivable," from stem of cogitare "to think" (see cogitation).
incohesive (adj.): 1826, from in- (1) "not" + cohesive. Related: Incohesively; incohesiveness.
incline (n.): c. 1600, "mental tendency," from incline (v.). The literal meaning "slant, slope" is attested from 1846 in railroading.
incommodity (n.): early 15c., from Old French incommodité (14c.), from Latin incommoditas "inconvenience, disadvantage; damage, injury," from incommodus "inconvenient, unsuitable, troublesome," from in- "not" (see in- (1)) + commodus "suitable, convenient" (see commode).
incomprehension (n.): c. 1600, from in- (1) "not" + comprehension.
inconsequence (n.): 1580s, from Late Latin inconsequentia, from Latin inconsequens "inconsequent" (see inconsequent).
inconsideration (n.): 1520s, "indiscretion, rashness, failure to consider," from Late Latin inconsiderationem (nominative inconsideratio) "inconsiderateness," from Latin inconsideratus "headstrong, thoughtless" (see inconsiderate).
incommensurability (n.): 1560s, from incommensurable + -ity.
incommunicability (n.): 1630s, from incommunicable + -ity.
incompatibility (n.): 1610s, from incompatible + -ity, or from French incompatibilité (15c.).
incompletion (n.): 1780, "incomplete condition," noun of state from incomplete. In reference to football passes, by 1926.
incomprehensibility (n.): 1590s, from incomprehensible + -ity; perhaps modeled on French incompréhensibilité (16c.).
inconstancy (n.): 1520s of persons, "fickleness;" 1610s of things, "mutability, irregularity," from Latin inconstantia "inconstancy, fickleness," noun of quality from inconstans "changeable, inconsistent" (see inconstant).