Online Etymology Dictionary

impudence (n.): late 14c., from Latin impudentia "shamelessness," noun of quality from impudens "shameless" (see impudent).
impudent (adj.): late 14c., from Latin impudentem (nominative impudens) "without shame, shameless," from assimilated form of in- "not, opposite of" (see in- (1)) + pudens "ashamed, modest," present-participle adjective from pudere "to cause shame" (see pudendum). Related: Impudently.
impugn (v.): "attack by argument," late 14c., from Old French impugner (14c.), from Latin impugnare "to fight against, assault, attack," from assimilated form of in- "into, in, on, upon" (see in- (2)) + pugnare "to fight" (see pugnacious). Related: Impugned; impugning. Impugnable has meant "liable to be assailed" (1823) and "that cannot be assailed" (1560s).
impulse (n.): early 15c., "an act of impelling, a thrust, push," from Latin impulsus "a push against, pressure, shock," figuratively "incitement, instigation," past participle of impellere "to strike against, push against" (see impel). Meaning "a stimulus in the mind to action, arising from some state or feeling" is first recorded 1640s. As an adjective, in reference to purchases made on impulse, 1955 (in impulse buyer).
impulsion (n.): early 15c., "a driving, pushing, thrusting," from Old French impulsion (14c.), from Latin impulsionem (nominative impulsio) "external pressure," figuratively "incitement, instigation," noun of action from past participle stem of impellere (see impel).
impulsive (adj.): early 15c., originally in reference to medicine that reduces swelling or humors, from Medieval Latin impulsivus, from Latin impuls-, past participle stem of impellere "strike against, push against" (see impel). Meaning "having the property of impelling" (of force, cause, energy, etc.) is from c. 1600. Of persons, "rash, characterized by impulses," from 1847, from impulse. Earlier, at least once, in reference to maniacs:
The impulsive insane are often irritable, restless and jealous. Sometimes they have delusions, and sometimes not. Their delusions frequently seem to have no connection with their outbreaks of violence. They are often the best and at the same time the most dangerous class of patients in the asylums. They have little of the charity of the world, are most likely to be punished for their offences, and yet have the least control over their conduct. ["Impulsive and Homicidal Insanity," Boston Medical and Surgical Journal," April 19, 1843]
impulsively (adv.): 1751, from impulsive + -ly (2).
impulsiveness (n.): 1650s, from impulsive + -ness.
impulsivity (n.): 1891; see impulsive + -ity.
impune (adj.): "unpunished" (obsolete), 1610s, from Latin impunis "unpunished" (see impunity). For the word meaning "attack by argument," see impugn.
impunity (n.): 1530s, from Middle French impunité (14c.) and directly from Latin impunitatem (nominative impunitas) "freedom from punishment, omission of punishment," also "rashness, inconsideration," from impunis "unpunished, without punishment," from assimilated form of in- "not, opposite of" (see in- (1)) + poena "punishment" (see penal).
impure (adj.): mid-15c., of wine, "muddy, not clear," from Middle French impur (13c.), from Latin impurus "not pure, unclean, filthy, foul," from assimilated form of in- "not, opposite of" (see in- (1)) + purus "pure" (see pure).

In English, the subsequent order of sense appearance seems to be "earthly, mundane, not spiritual" (c. 1500); "obscene, lewd, unchaste, immoral" (1530s); "mixed with offensive matter, tainted" (1590s); "mixed or combined with other things" (without reference to foulness), 1620s. As a noun from 1784. Related: Impurely.
Impuritan (n.): "one who is not a Puritan," 1610s, a hostile coinage of the Puritans, from im- "not, opposite of" + Puritan, perhaps also with suggestion of impure.
impurity (n.): mid-15c., "thing which makes or is impure;" c. 1500, "fact or quality of being impure," from Middle French impurité, from impur (see impure). Related: Impurities.
imputable (adj.): 1620s, from Medieval Latin imputabilis, from Latin imputare "to charge, ascribe" (see impute). Related: Imputability.
imputation (n.): 1540s, noun of action from impute (v.) on model of Middle French imputation, or else from Late Latin imputationem (nominative imputatio) "a charge, an account," noun of action from Latin imputare "to charge, ascribe."
impute (v.): early 15c., from Old French imputer, emputer (14c.) and directly from Latin imputare "to reckon, make account of, charge, ascribe," from assimilated form of in- "in, into" (see in- (2)) + putare "to trim, prune; reckon, clear up, settle (an account)," from PIE *puto- "cut, struck," suffixed form of root *pau- (2) "to cut, strike, stamp" (see pave). Related: Imputed; imputing.
in (adv., prep.): a Middle English merger of Old English in (prep.) "in, into, upon, on, at, among; about, during;" and Old English inne (adv.) "within, inside," from Proto-Germanic *in (source also of Old Frisian, Dutch, German, Gothic in, Old Norse i), from PIE *en "in" (source also of Greek en, Latin in "in, into," Old Irish in, Welsh yn, Old Church Slavonic on-). The simpler form took on both senses in Middle English.

Sense distinction between in and on is from later Middle English, and nuances in use of in and at still distinguish British and American English (in school/at school). Sometimes in Middle English shortened to i.

The noun sense of "influence, access (to power or authorities)," as in have an in with, is first recorded 1929 in American English. to be in for it "certain to meet with something unpleasant" is from 1690s. To be in with "on friendly terms with" is from 1670s. Ins and outs "intricacies, complications of an action or course" is from 1660s. In-and-out (n.) "copulation" is attested from 1610s.
in (adj.): "that is within, internal," 1590s, from in (adv.). Sense of "holding power" (the in party) first recorded c. 1600; that of "exclusive" (the in-crowd, an in-joke) is from 1907 (in-group); that of "stylish, fashionable" (the in thing) is from 1960.
in absentia (adv.): Latin, literally "in (his/her/their) absence" (see absence). By 1831 in English, earlier in legal Latin.
in custodia legis (adv.): legal Latin, "in the custody of the law," from ablative of custodia "a guarding, watching, keeping" (see custody) + legis, genitive of lex "law" (see legal (adj.)).
in extremis: "at the point of death," 16c., Latin, literally "in the farthest reaches," from ablative plural of extremus "extreme" (see extreme (adj.)).
in facie curiae: "before the court," legal Latin, from ablative of Latin facies "form, face" (see face (n.)). + genitive of curia "court" (see curia).
in fieri: legal Latin, "in the process of being done," from fieri "to come into being, become," used as passive of facere "to make, do" (see factitious).
in forma pauperis: legal Latin, literally "in the form of a poor person" (thus exempt from certain court fees, etc.), 1590s; see form (n.) + pauper (n.).
in loco parentis: legal Latin, 1640s in English, literally "in the place of a parent," from loco, ablative of locus "a place" (see locus (n.)) + parentis, genitive of parens "parent" (see parent (n.)).
in medias res: Latin, literally "in the midst of things," from medias, accusative fem. plural of medius "middle" (see medial (adj.)) + accusative plural of res "a thing," from PIE *re- "to bestow, endow." From Horace, in reference to narrative technique:
Semper ad eventum festinat, et in medias res,
Non secus ac notas auditorem rapit (etc.)
in memoriam: Latin, literally "in memory of," from accusative of memoria "memory" (see memory). The phrase was much-used in Latin writing; Tennyson's poem of that name (published in 1850) seems to have introduced the phrase to English.
in re (prep.): "in the matter of, in the (legal) case of," c. 1600, probably from Duns Scotus; Latin, from re, ablative of res "property, goods; matter, thing, affair," from Proto-Italic *re-, of uncertain origin. According to de Vaan from PIE *Hreh-i- "wealth, goods" (cognates Sanskrit rayi- "property, goods," Avestan raii-i- "wealth").
in situ: 1740, Latin, literally "in its (original) place or position," from ablative of situs "site" (see site (n.)).
in totidem verbis: Latin phrase, "in just so many words," that is, "in these very words," from demonstrative of Latin totus "whole, entire" (see total (adj.)) + ablative plural of verbum "word" (see verb).
in toto (adv.): Latin, "as a whole, wholly, completely, utterly, entirely," from toto, ablative of totus "whole, entire" (see total (adj.)); "always or nearly always with verbs of negative sense" [Fowler].
in utero: 1713, Latin, literally "in the uterus," from ablative of uterus (see uterus).
in vitro: 1892, scientific Latin; "in a test tube, culture dish, etc.;" literally "in glass," from Latin vitrum "glass" (see vitreous).
in vivo: 1898, Latin; "within a living organism," from vivere "to live" (see vital).
in't: archaic or poetic contraction of in it, attested from 17c. I'nt, also i'n't, as a contraction of is not is recorded from 1742.
in- (1): word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne "not" (see un- (1)).

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
in- (2): element meaning "into, in, on, upon" (also im-, il-, ir- by assimilation of -n- with following consonant), from Latin in- "in" (see in).

In Old French (and hence in Middle English) this often became en-, which in English had a strong tendency to revert to Latin in-, but not always, which accounts for pairs such as enquire/inquire. There was a native form, which in West Saxon usually appeared as on- (as in Old English onliehtan "to enlighten"), and some of those verbs survived into Middle English (such as inwrite "to inscribe"), but all now seem to be extinct.

Not related to in- (1) "not," which also was a common prefix in Latin, causing confusion: to the Romans impressus could mean "pressed" or "unpressed;" inaudire meant "to hear," but inauditus meant "unheard of;" in Late Latin investigabilis could mean "that may be searched into" or "that cannot be searched into." Latin invocatus was "uncalled, uninvited," but invocare was "to call, appeal to."

The trouble has continued in English; the hesitation over what is meant by inflammable being a commonly cited example. Implume (1610s) meant "to feather," but implumed (c. 1600) meant "unfeathered." Impliable can mean "capable of being implied" (1865) or "inflexible" (1734). Impartible in 17c. could mean "incapable of being divided" or "capable of being imparted." Impassionate can be "free from passion" or it can mean "strongly stirred by passion." Inanimate (adj.) is "lifeless," but Donne uses inanimate (v.) to mean "infuse with life or vigor." Irruption is "a breaking in," but irruptible is "unbreakable." In addition to improve "use to one's profit," Middle English also had a verb improve meaning "to disprove" (15c.). To inculpate is "to accuse," but inculpable means "not culpable, free from blame." Infestive has meant "troublesome, annoying" (1560s, from infest) and "not festive" (1620s). In Middle English inflexible could mean "incapable of being bent" or "capable of being swayed or moved." In 17c., informed could mean "current in information," formed, animated," or "unformed, formless" ("This was an awkward use" [OED]). Inhabited has meant "dwelt in" (1560s) and "uninhabited" (1610s); inhabitable likewise has been used on opposite senses, a confusion that goes back to Late Latin.
in-between (n.): 1815, "an interval;" also "a person who intervenes," noun use of prepositional phrase, from in (adv.) + between. Related: In-betweener (1912); in-betweenity (1927).
in-country (n.): "interior regions" of a land, 1560s, from in (prep.) + country.
in-fighting (n.): 1816, in pugilism, the practice of getting at close quarters with an opponent; see in + fighting. Old English infiht (n.) meant "brawl within a house or between members of a household." Middle English had infight (v.) "to attack" (c. 1300); the modern verb infight "fight at close quarters" (1916) appears to be a back-formation from in-fighting. Related: In-fighter (1812).
in-flight (adj.): also inflight, "during or within a flight," 1945, from in (prep.) + flight.
in-gather (v.): also ingather, 1570s, from in (adv.) + gather (v.). Related: Ingathered; ingathering (1530s).
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.
in-house (adj.): also inhouse, 1955, from in (prep.) + house (n.).
in-itselfness (n.): 1879, in philosophy; see in (adv.) + itself + -ness.
in-joke (n.): 1964, from in (adj.) + joke (n.).
in-law (n.): 1894, "anyone of a relationship not natural," abstracted from father-in-law, etc.
The position of the 'in-laws' (a happy phrase which is attributed ... to her Majesty, than whom no one can be better acquainted with the article) is often not very apt to promote happiness. ["Blackwood's Magazine," 1894]
The earliest recorded use of the formation is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited. Thus the word originally had a more narrow application; its general extension to more distant relatives of one's spouse is, according to OED "recent colloquial or journalistic phraseology." Middle English inlaue (13c.) meant "one within or restored to the protection and benefit of the law" (opposite of an outlaw), from a verb inlauen, from Old English inlagian "reverse sentence of outlawry."
in-migration (n.): 1942, American English, in reference to movement within the same country (as distinguished from immigration), from in (prep.) + migration.
in-patient (n.): also inpatient, 1760, "person who stays in a hospital for treatment," from in (prep.) + patient (n.). As an adjective by 1890.