igneous (adj.) Look up igneous at Dictionary.com
1660s, from Latin igneus "of fire, fiery," from ignis "fire," from PIE *egni- "fire" (cognates: Sanskrit agnih "fire, sacrificial fire," Old Church Slavonic ogni, Lithuanian ugnis "fire").
ignis fatuus (n.) Look up ignis fatuus at Dictionary.com
"will o' the wisp, jack-a-lantern," 1560s, from Medieval Latin, literally "foolish fire;" see igneous + fatuous.
ignitable (adj.) Look up ignitable at Dictionary.com
1640s; see ignite + -able.
ignite (v.) Look up ignite at Dictionary.com
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.) Look up ignition at Dictionary.com
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.) Look up ignivomous at Dictionary.com
"vomiting fire," c.1600, from Late Latin ignivomous, from Latin ignis "fire" (see igneous) + vomere "to vomit" (see vomit).
ignoble (adj.) Look up ignoble at Dictionary.com
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.) Look up ignominious at Dictionary.com
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.) Look up ignominy at Dictionary.com
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.) Look up ignoramus at Dictionary.com
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.) Look up ignorance at Dictionary.com
c.1200, from Old French ignorance (12c.), from Latin ignorantia "want of knowledge" (see ignorant).
ignorant (adj.) Look up ignorant at Dictionary.com
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" (cognates: 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." Also see uncouth. Colloquial sense of "ill-mannered" first attested 1886. As a noun meaning "ignorant person" from mid-15c.
ignore (v.) Look up ignore at Dictionary.com
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.) Look up iguana at Dictionary.com
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.) Look up Iguanodon at Dictionary.com
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.) Look up ikebana at Dictionary.com
1901, from Japanese, from ikeru "to keep alive, arrange" + hana "flower."
il- Look up il- at Dictionary.com
assimilated form of Latin prefix in- used with words beginning in l-; see in-.
ileo- Look up ileo- at Dictionary.com
comb. form from ileum (q.v.).
ileum (n.) Look up ileum at Dictionary.com
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.) Look up ileus at Dictionary.com
painful intestinal condition, 1706, from Latin ileus "severe colic," from Greek ileos "colic," from eilein "to turn, squeeze," from PIE *wel- (3) "to turn, roll" (see volvox).
ilex (n.) Look up ilex at Dictionary.com
late 14c., from Latin ilex "holm-oak," perhaps from an extinct non-Indo-European language.
ilia Look up ilia at Dictionary.com
Latin plural of ilium (see ileum).
iliac (adj.) Look up iliac at Dictionary.com
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 Look up Iliad at Dictionary.com
from Latin Ilias (genitive Iliadis), from Greek Ilias poiesis "poem of Ilion" (Troy), literally "city of Ilius," the mythical founder.
ilium (n.) Look up ilium at Dictionary.com
pelvic bone, 1706, Modern Latin, from Latin ilia (plural) "groin, flank" (see ileum).
ilk (adj.) Look up ilk at Dictionary.com
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.) Look up ill at Dictionary.com
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.) Look up ill at Dictionary.com
early 13c., "to do evil to," from ill (adj.). Meaing "to speak disparagingly" is from 1520s. Related: Illed; illing.
ill (adv.) Look up ill at Dictionary.com
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.) Look up illative at Dictionary.com
"inferential," 1610s, from Late Latin illativus, from Latin illatus "brought in," used as past participle of inferre. As a noun from 1590s.
illegal (adj.) Look up illegal at Dictionary.com
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.) Look up illegality at Dictionary.com
1630s, from illegal + -ity; or else from French illegalité (14c.).
illegally (adv.) Look up illegally at Dictionary.com
1620s, from illegal + -ly (2).
illegible (adj.) Look up illegible at Dictionary.com
1630s, from assimilated form of in- (1) "not, opposite of" + legible. Related: Illegibly; illegibility.
illegitimacy (n.) Look up illegitimacy at Dictionary.com
1670s; see illegitimate + -acy.
illegitimate (adj.) Look up illegitimate at Dictionary.com
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.) Look up illiberal at Dictionary.com
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.) Look up illicit at Dictionary.com
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.) Look up illimitable at Dictionary.com
1590s, from il- "not" + limitable.
illing (n.) Look up illing at Dictionary.com
"evil-doing, malevolent treatment," early 13c.; see ill (v.).
Illinois Look up Illinois at Dictionary.com
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.) Look up illiquid at Dictionary.com
1690s, from il- "not" + liquid in the financial sense.
illiteracy (n.) Look up illiteracy at Dictionary.com
1650s, from illiterate + -cy. Earlier in this sense was illiterature (1590s).
illiterate (adj.) Look up illiterate at Dictionary.com
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). Rendered in Old English as unstæfwis. As a noun meaning "illiterate person" from 1620s. Hence, illiterati (1788).
illness (n.) Look up illness at Dictionary.com
"disease, sickness," 1680s, from ill + -ness. Earlier it meant "bad moral quality" (c.1500).
illocution (n.) Look up illocution at Dictionary.com
1955, from assimilated form of in- (1) "not, opposite of" + locution.
illocutionary (adj.) Look up illocutionary at Dictionary.com
1955, from illocution + -ary.
illogical (adj.) Look up illogical at Dictionary.com
1580s, from assimilated form of in- (1) "not, opposite of" + logical. Related: Illogically.
illth (n.) Look up illth at Dictionary.com
1867, coined by John Ruskin from ill (adv.) on model of wealth (also see -th (2)).
[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.) Look up illude at Dictionary.com
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).