Online Etymology Dictionary

igneous (adj.): 1660s, from Latin igneus "of fire, fiery," from ignis "fire," from PIE *egni- "fire" (cf. Sanskrit agnih "fire, sacrificial fire," Old Church Slavonic ogni, Lithuanian ugnis "fire").
ignis fatuus (n.): "will o' the wisp, jack-a-lantern," 1560s, from Medieval Latin, literally "foolish fire;" see igneous + fatuous.
ignitable (adj.): 1640s; see ignite + -able.
ignite (v.): 1660s, from Latin ignitus, past participle of ignire "set on fire," from ignis "fire" (see igneous). Attested earlier as an adjective (1550s). Related: Ignited; igniting.
ignition (n.): 1610s, "act of heating to the point of combustion," from French ignition (16c.) or directly from Medieval Latin ignitionem (nominative ignitio), from Latin ignire "set on fire," from ignis "fire" (see igneous). Meaning "means of sparking an internal combustion engine" is from 1881.
ignivomous (adj.): "vomiting fire," c.1600, from Late Latin ignivomous, from Latin ignis "fire" (see igneous) + vomere "to vomit" (see vomit).
ignoble (adj.): mid-15c., "of low birth," from Middle French ignoble, from Latin ignobilis "unknown, undistinguished, obscure; of base birth, not noble," from assimilated form of in- "not, opposite of" (see in- (1)) + gnobilis "well-known, famous, renowned, of superior birth" (see noble). Related: Ignobly.
ignominious (adj.): early 15c., from Middle French ignominieux (14c.) or directly from Latin ignominiosus "disgraceful, shameful," from ignominia "loss of a (good) name," from assimilated form of in- "not, opposite of" (see in- (1)) + nomen (genitive nominis) "name" (see name). Influenced by Old Latin gnoscere "come to know." Related: Ignominiously; ignominiousness.
ignominy (n.): 1530s, back-formation from ignominious or else from Middle French ignominie (15c.), from Latin ignominia "disgrace, dishonor" (see ignominious). Also sometimes shortened to ignomy.
ignoramus (n.): 1570s, from an Anglo-French legal term (early 15c.), from Latin ignoramus "we do not know," first person present indicative of ignorare "not to know" (see ignorant). The legal term was one a grand jury could write on a bill when it considered the prosecution's evidence insufficient. Sense of "ignorant person" came from the title role of George Ruggle's 1615 play satirizing the ignorance of common lawyers.
ignorance (n.): c.1200, from Old French ignorance (12c.), from Latin ignorantia "want of knowledge" (see ignorant).
ignorant (adj.): late 14c., from Old French ignorant (14c.), from Latin ignorantia, from ignorantem (nominative ignorans), present participle of ignorare "not to know, to be unacquainted; mistake, misunderstand; take no notice of, pay no attention to," from assimilated form of in- "not, opposite of" (see in- (1)) + Old Latin gnarus "aware, acquainted with" (cf. Classical Latin noscere "to know," notus "known"), from Proto-Latin suffixed form *gno-ro-, related to gnoscere "to know" (see know).

Form influenced by Latin ignotus "unknown." Cf. also uncouth. Colloquial sense of "ill-mannered" first attested 1886. As a noun meaning "ignorant person" from mid-15c.
ignore (v.): 1610s, "not to know, to be ignorant of," from French ignorer "be unaware of," from Latin ignorare "not to know, disregard" (see ignorant). Sense of "pay no attention to" first recorded 1801 (Barnhart says "probably a dictionary word"), and not common until c.1850. Related: Ignored; ignoring.
iguana (n.): 1550s, from Spanish, from Arawakan (W.Indies) iguana, iwana, the local name for the lizard.
Foure footed beastes ... named Iuannas, muche lyke vnto Crocodiles, of eyght foote length, of moste pleasaunte taste. [Richard Eden, "Decades of the New World," 1555]
Iguanodon (n.): dinosaur name, 1825, hybrid from iguana + stem of Greek odonys "tooth" (on model of mastodon). So called because the fossil teeth and bones were thought to resemble those of the lizard.
ikebana (n.): 1901, from Japanese, from ikeru "to keep alive, arrange" + hana "flower."
il-: assimilated form of Latin prefix in- used with words beginning in l-; see in-.
ileo-: comb. form from ileum (q.v.).
ileum (n.): lowest part of the small intestine, 1680s, medical Latin, from ileum, singular created from classical Latin plural ilia "groin, flank," in classical Latin, "belly, the abdomen below the ribs," poetically, "entrails, guts." Sense restriction and form apparently from confusion with Greek eileos (see ileus). Earlier in English ylioun (late 14c.), from Medieval Latin ileon. Related: Ileitis.
ileus (n.): painful intestinal condition, 1706, from Latin ileus "severe colic," from Greek ileos "colic," from eilein "to turn, squeeze," from PIE *wel- "to turn, roll" (see vulva).
ilex (n.): late 14c., from Latin ilex "holm-oak," perhaps from an extinct non-Indo-European language.
ilia: Latin plural of ilium (see ileum).
iliac (adj.): 1510s, "pertaining to the disease ileus or colic," from French iliaque or directly from Late Latin iliacus, from ilium "flank, side, entrails" (see ileum).
Iliad: from Latin Ilias (genitive Iliadis), from Greek Ilias poiesis "poem of Ilion" (Troy), literally "city of Ilius," the mythical founder.
ilium (n.): pelvic bone, 1706, Modern Latin, from Latin ilia (plural) "groin, flank" (see ileum).
ilk: Old English ilca "same" (n. and adj.), from Proto-Germanic *ij-lik, in which the first element is from the PIE demonstrative particle *i- (see yon) and the second is that in Old English -lic "form" (see like). Of similar formation are which and such. Phrase of that ilk implies coincidence of name and estate, as in Lundie of Lundie; applied usually to families, so by c.1790 it began to be used with meaning "family," then broadening to "type, sort."
ill (adj.): c.1200, "morally evil" (other 13c. senses were "malevolent, hurtful, unfortunate, difficult"), from Old Norse illr "ill, bad," of unknown origin. Not related to evil. Main modern sense of "sick, unhealthy, unwell" is first recorded mid-15c., probably related to Old Norse idiom "it is bad to me." Slang inverted sense of "very good, cool" is 1980s. As a noun, "something evil," from mid-13c.
ill (v.): early 13c., "to do evil to," from ill (adj.). Meaing "to speak disparagingly" is from 1520s. Related: Illed; illing.
ill (adv.): c.1200, "wickedly; with hostility;" see ill (adj.). Meaning "not well, poorly" is from c.1300. It generally has not shifted to the realm of physical sickess, as the adjective has done. Ill-fated recorded from 1710; ill-informed from 1824; ill-tempered from c.1600; ill-starred from c.1600. Generally contrasted with well, hence the useful, but now obsolete or obscure illcome (1570s), illfare (c.1300), and illth.
illative (adj.): "inferential," 1610s, from Late Latin illativus, from Latin illatus "brought in," used as past participle of inferre. As a noun from 1590s.
illegal (adj.): 1620s, from French illégal or directly from Medieval Latin illegalis, from assimilated form of in- "not, opposite of" (see in- (1)) + Latin legalis (see legal). Term illegal immigrant first recorded 1892 in American English (illegal immigration is from 1887).
illegality (n.): 1630s, from illegal + -ity; or else from French illegalité (14c.).
illegally (adv.): 1620s, from illegal + -ly (2).
illegible (adj.): 1630s, from assimilated form of in- (1) "not, opposite of" + legible. Related: Illegibly; illegibility.
illegitimacy (n.): 1670s; see illegitimate + -acy.
illegitimate (adj.): 1530s, "born out of wedlock," formed in English (and replacing earlier illegitime, c.1500), modeled on Latin illegitimus "not legitimate" (see il- + legitimate). Sense of "unauthorized, unwarranted" is from 1640s. Phrase illegitimi non carborundum, usually "translated" as "don't let the bastards grind you down," is fake Latin (by 1965, said to date from c.1939) (Carborundum was a brand of abrasives).
illiberal (adj.): 1530s, "ungentlemanly, base, mean," from Middle French illiberal (14c.), from Latin illiberalis "ungenerous, mean, sordid; unworthy of a freeman," from assimilated form of in- "not, opposite of" (see in- (1)) + liberalis (see liberal). A sense of "narrow-minded politically; unconcerned with the rights or liberties of others" is attested from 1640s, and might conceivably be revived to take up some of the burden that drags down conservative.
illicit (adj.): c.1500, from Old French illicite (14c.) "unlawful, forbidden," from Latin illicitus "not allowed, unlawful, illegal," from assimilated form of in- "not, opposite of" (see in- (1)) + licitus "lawful," past participle of licere "to be allowed" (see licence). Related: Illicitly.
illimitable (adj.): 1590s, from il- "not" + limitable.
illing: "evil-doing," early 13c.; see ill (v.).
Illinois: U.S. territory created 1809; name is from a native Algonquian people who called themselves Inoca (1725), also written Ilinouek, Old Ottawa for "ordinary speaker." The modern form represents a 17c. French spelling, pronounced "ilinwe" at that time. Admitted as a state 1818.
illiquid (adj.): 1690s, from il- "not" + liquid in the financial sense.
illiteracy (n.): 1650s, from illiterate + -cy. Earlier in this sense was illiterature (1590s).
illiterate (adj.): early 15c., "uneducated, unable to read (originally of Latin)," from Latin illiteratus "unlearned, unlettered, ignorant; without culture, inelegant," from assimilated form of in- "not, opposite of" (see in- (1)) + literatus, literally "furnished with letters" (see literate). As a noun meaning "illiterate person" from 1620s. Hence, illiterati (1788).
illness (n.): "disease, sickness," 1680s, from ill + -ness. Earlier it meant "bad moral quality" (c.1500).
illocution (n.): 1955, from assimilated form of in- (1) "not, opposite of" + locution.
illocutionary: 1955, from illocution + -ary.
illogical (adj.): 1580s, from assimilated form of in- (1) "not, opposite of" + logical. Related: Illogically.
illth (n.): 1867, coined by John Ruskin from ill (adv.) on model of wealth (also see -th).
[S]uch things, and so much of them as he can use, are, indeed, well for him, or Wealth; and more of them, or any other things, are ill for him, or Illth. [Ruskin, "Munera Pulveris"]
illude (v.): early 15c., "to mock, to trick," from Latin illudere "to make sport of," from assimilated form of in- "in, into" (see in- (2)) + ludere "to play" (see ludicrous).