Online Etymology Dictionary

ingrain (v.): see engrain, or ingrained.
ingrained (adj.): "deeply rooted," 1590s, literally "dyed with grain "cochineal," the red dyestuff (see engrain). Figuratively, "thoroughly imbued" (of habits, principles, prejudices, etc.) from 1851. In reference to dyed carpets, etc., it is attested from 1766, from the manufacturing phrase in (the) grain "in the raw material before manufacture."
ingrate (n.): "ungrateful person," 1670s, from earlier adjective meaning "unfriendly," also "ungrateful, unthankful" (14c.), from Latin ingratus "unpleasant, disagreeable," also "ungrateful, unthankful," and "thankless, unprofitable," from in- "not" (see in- (1)) + gratus "pleasing, beloved, dear, agreeable" (see grace (n.)).
ingratiate (v.): 1620s, possibly via 16c. Italian ingraziarsi "to bring (oneself) into favor," or an unrecorded Medieval Latin *ingratiatus, from Latin phrase in gratiam "for the favor of," from in "in" (see in- (2)) + gratia "favor, grace" (see grace (n.)). Related: Ingratiated; ingratiating.
ingratiation (n.): 1804, noun of action from ingratiate (v.).
ingratitude (n.): mid-14c., from Old French ingratitude "ungratefulness" (13c.) and directly from Late Latin ingratitudinem (nominative ingratitudo) "unthankfulness," noun of quality from Latin ingratus "ungrateful" (see ingrate).
ingrave (v.): see engrave. Related: Ingraved; ingraving.
ingredient (n.): in early use also engredient, early 15c., "something forming part of a mixture," from Latin ingredientem (nominative ingrediens) "that which enters into" (a compound, recipe, etc.), present participle of ingredi "go in, enter," from in- "in" (see in- (2)) + gradi "to step, go" (see grade (n.)). Also from early 15c. as an adjective, "forming part of a mixture."
ingress (n.): mid-15c., from Latin ingressus "an advance; walking; an entry," from ingress-, past participle stem of ingredi "to step into, enter," from in- "in" (see in- (2)) + gradi "to step, go" (see grade (n.)). The verb meaning "to enter, go in" sometimes said to be American English, but it is attested from early 14c.
Ingrid: fem. proper name, Scandinavian or German, from Ing, Germanic god-name (Old Norse Yngvi, Old English Ingwine), apparently an earlier name of Freyr. He was associated with prosperity, virility, and fertility. Second element in the name is either friðr "fair, beautiful" or rida "to ride." As a given name for girls in the U.S., almost unknown before 1940 (about the time Swedish actress Ingrid Bergman rose to fame in Hollywood); it was most popular in 1960s and early '70s but never common.
ingrown (adj.): also in-grown, 1660s, "native, innate," from in + grown. Of nails, "that has grown into the flesh," 1869 (in-growing in this sense is from 1847). Related: Ingrowth (1870).
inguinal (adj.): 1680s, from French inguinal (16c.) or directly from Latin inguinalis "of the groin," from inguen (genitive inguinis) "groin," from PIE *engw- "groin, internal organ" (source also of Greek aden "gland"). Related: Inguinally.
ingurgitation (n.): "immoderate eating and drinking," 1520s, from Late Latin ingurgitationem (nominative ingurgitatio), noun of action from past participle stem of Latin ingurgitare "plunge into, gorge," from in- "in, into" (see in- (2)) + gurgitare "to engulf," from gurges "whirlpool, gorge" (see gurges).
Ingvaeonic (n.): hypothetical ancestral North Sea Germanic language, 1933, from Latin Ingaeuones, name of a Germanic tribe in Tacitus, literally "people of Yngve," god, demigod, or eponymous ancestor. Earlier the word was used in English in reference to North Sea Germanic tribes (1904).
inhabit (v.): late 14c., from Old French enhabiter, enabiter "dwell in, live in, reside" (12c.), from Latin inhabitare "to dwell in," from in- "in" (see in- (2)) + habitare "to dwell," frequentative of habere "hold, have" (see habit (n.)). Formerly also enhabit. Related: Inhabited; inhabiting.
inhabitable (adj.): 1. "not habitable," late 14c., from Old French inhabitable (14c.), from Latin inhabitabilis, from in- "not, opposite of" (see in- (1)) + habitabilis (see habitable).

2. "capable of being inhabited" (the main modern sense), c. 1600, from inhabit + -able). In Late Latin, inhabitabilis also was used in a sense of "that can be inhabited." A word used in two opposite senses.
inhabitant (n.): "one who dwells in a place" (as distinguished from a visitor or transient), early 15c., from Anglo-French inhabitant, from Latin inhabitantem (nominative inhabitans), present participle of inhabitare "to dwell in" (see inhabit). Related: Inhabitants. As an adjective, also from early 15c.
inhalant (adj.): 1804, from Latin inhalantem, present participle of inhalare (see inhale). As a noun from 1830.
inhalation (n.): 1620s, "a breathing in," noun of action from past participle stem of Latin inhalare "breathe upon" (used here as if it meant "to breathe in"), from in- "on, upon" (see in- (2)) + halare "breathe."
inhale (v.): 1725, "to breathe in, draw air into the lungs," a back-formation from inhalation or else from French inhaler in this sense; used as a word to be the opposite of exhale. Slang sense of "eat rapidly" is recorded from 1924. As a noun, "act of inhaling," by 1904. Related: Inhaled; inhaling.
inhance (v.): obsolete form of enhance. Related: Inhancement.
inharmonious (adj.): 1711, from in- (1) "not" + harmonious. Related: Inharmoniously.
inhere (v.): 1580s, "to exist or have being" (in something), "belong to the intrinsic nature of," from Latin inhaerere "to stick in or to," also figurative (see inherent). Related: Inhered; inhering.
inherence (n.): 1570s, from Middle French inhérence (15c.) or directly from Medieval Latin inhaerentia, from inhaerentem (see inherent). Related: Inherency (c. 1600).
inherent (adj.): 1570s, from Latin inhaerentem (nominative inhaerens), present participle of inhaerere "be closely connected with, be inherent," literally "adhere to, cling to," from in- "in" (see in- (2)) + haerere "to stick" (see hesitation). Related: Inherently.
inherit (v.): c. 1300, "to make (someone) an heir" (a sense now obsolete), from Old French enheriter "make heir, attribute the right of inheretance to, appoint as heir," from Late Latin inhereditare "to appoint as heir," from in- "in" (see in- (2)) + Latin hereditare "to inherit," from heres (genitive heredis) "heir" (see heredity).

Sense of "receive inheritance, get by succession as representative of the former possessor" is attested from mid-14c.; in Medieval Latin inhereditare also had taken on a sense "put in possession." Original sense is retained in disinherit. Related: Inherited; inheriting; inheritable.
inheritance (n.): late 14c., enheritaunce "fact of receiving by hereditary succession;" early 15c. as "that which is or may be inherited," from Anglo-French and Old French enheritaunce, from Old French enheriter "make heir, appoint as heir" (see inherit). Heritance "act of inheriting" is from mid-15c.
inhesion (n.): 1630s, from Late Latin inhaesionem (nominative inhaesio) "a hanging or adhering to," noun of action from past participle stem of Latin inhaerere "to stick in or into" (see inherent).
inhibit (v.): early 15c., "to forbid, prohibit," back-formation from inhibition or else from Latin inhibitus, past participle of inhibere "to hold in, hold back, keep back." Psychological sense (1876) is from earlier, softened meaning of "restrain, check, hinder" (1530s). Related: Inhibited; inhibiting.
inhibition (n.): late 14c., "formal prohibition; interdiction of legal proceedings by authority;" also, the document setting forth such a prohibition, from Old French inibicion and directly from Latin inhibitionem (nominative inhibitio) "a restraining," from past participle stem of inhibere "to hold in, hold back, keep back," from in- "in, on" (see in- (2)) + habere "to hold" (see habit (n.)). Psychological sense of "involuntary check on an expression of an impulse" is from 1876.
inhibitor (n.): 1868 as a Scottish legal term; 1914 in biochemistry; agent noun in Latin form from inhibit. Form inhibiter is attested from 1610s.
inhibitory (adj.): late 15c., from Medieval Latin inhibitorius "inhibitory," from inhibit-, past participle stem of Latin inhibere "to hold in, keep back" (see inhibition).
inhospitable (adj.): 1560s, from Middle French inhospitable (15c.), from Medieval Latin inhospitabilis (equivalent of Latin inhospitalis), from in- "not" (see in- (1)) + Medieval Latin hospitabilis (see hospitable).
inhuman (adj.): mid-15c., "cruel," from Latin inhumanus "inhuman, savage, cruel, rude, barbarous," from in- "not, opposite of" (see in- (1)) + humanus "human" (see human). Spelled inhumane till 18c. (see humane).
inhumane (adj.): originally a variant spelling and pronunciation of inhuman "cruel, hard-hearted;" it appears to have died out 17c. but returned c. 1822, probably a reformation as a negative of humane (q.v.), with its accent.
inhumanity (n.): "barbarous cruelty," late 15c., from French inhumanité (14c.) or directly from Latin inhumanitatem (nominative inhumanitas) "inhuman conduct, savageness; incivility, rudeness," noun of quality from inhumanus "inhuman, savage, cruel" (see inhuman).
And Man, whose heav'n-erected face
The smiles of love adorn,--
Man's inhumanity to man
Makes countless thousands mourn!
[Robert Burns, "Man was Made to Mourn," 1784]
inhumation (n.): "act of burying in the ground" (as opposed to cremation), 1630s, noun of action from inhume "to bury" (see inhume).
inhume (v.): "bury, lay in the grave," c. 1600, from Latin inhumare "to bury," literally "to put into the ground," from in- "in" (see in- (2)) + humus "earth, soil" (see humus). Related: Inhumed; inhuming.
Inigo: masc. proper name, from Spanish Iñigo, probably from Latin Ignatius.
inimical (adj.): 1640s, from Late Latin inimicalis "hostile," from Latin inimicus "unfriendly; an enemy" (see enemy).
Inimical expresses both feeling and action, generally in private affairs. Hostile also expresses both feeling and action, but applies especially to public affairs: where it applies to private matters, it expresses either strong or conspicuous action or feeling, or both, or all. [Century Dictionary, 1902]
inimitability (n.): 1711, from inimitable + -ity. Perhaps from or modeled on French inimitabilité.
inimitable (adj.): late 15c., from Latin inimitabilis "that cannot be imitated," from in- "not, opposite of" (see in- (1)) + imitabilis "imitable" (see imitable). Related: Inimitably.
iniquitous (adj.): "unjust wicked," 1670s, from iniquity + -ous. Earlier were iniquous (1650s, from Latin iniquus) and inique (1520s, from French inique). Related: Iniquitously; iniquitousness.
iniquity (n.): c. 1300, "hostility, malevolence; a hostile action," from Old French iniquité, iniquiteit "wickedness; unfavorable situation" (12c.), from Latin iniquitatem (nominative iniquitas) "unequalness, unevenness," figuratively "unfavorableness, unfairness, injustice," noun of quality from iniquus "unjust, unequal; slanting, steep," from in- "not" (see in- (1)) + aequus "just, equal" (see equal (adj.)).

For the vowel change in the Latin compound, see acquisition. Meaning "evil, wickedness" is from late 14c. Old Iniquity (1610s) was a comic or buffoonish character in old morality plays, representing vice.
initial (adj.): 1520s, "of or pertaining to a beginning," from Middle French initial or directly from Latin initialis "initial, incipient, of the beginning," from initium "a beginning, a commencement; an entrance, a going in," noun use of neuter past participle of inire "to go into, enter upon, begin," from in- "into, in" (see in- (2)) + ire "to go" (see ion). Related: Initially.
initial (v.): "to mark or sign with initials," 1837, from initial (n.). Related: Initialed; initialing.
initial (n.): "initial letter of a name or surname," 1620s, from initial (adj.) in a specialized sense "standing at the beginning of a word, sentence, etc." (1620s).
initialese (n.): "abbreviation by use of initials," 1950, from initial (n.) + -ese.
initialism (n.): "written word formed from the first letters, in order, of other words in a name or phrase," 1957, from initial (n.) + -ism. The word was used earlier in a sense "group of initial letters of an author's name (rather than the full name) on a publication" (1868). An earlier term for what we now call an initialism was alphabetic abbreviation (1907).

The distinction from acronym is not universally agreed-upon; but in general, cases such as NATO, where the letters always are sounded as a word, are regarded as acronyms, those such as FBI, where the letters sound as letters, are initial-words or initialisms. The use of acronym in entries in this dictionary that are technically initialisms is a deliberate error, because many people will search all such words using "acronym."
initialize (v.): "to make ready for operation," 1957, from initial (adj.) + -ize. The same formation had been used earlier to mean "use initials instead of a name" (1837); "designate by initials" (1833). Related: Initialized; initializing; initialization (1957 in the modern sense).