Online Etymology Dictionary

in: O.E. in "in," inne "within," from P.Gmc. *in (cf. O.Fris, Du., Ger., Goth. in, O.N. i), from PIE *en (cf. Gk. en, L. in "in, into," O.Ir. in, Welsh yn-, O.C.S. on-). Sense of "holding power" (the in party) first recorded c.1600; that of "exclusive" (the in-crowd, an in-joke) is from 1907; that of "stylish, fashionable" (the in thing) is from 1960. The noun sense of "influence, access" (have an in with) first recorded 1929 in Amer.Eng. In-and-out "copulation" is attested from 1610s.
in absentia: Latin, lit. “in (his/her/their) absence” (see absence).
in like Flynn: 1940s slang, said to have originated in the U.S. military, perhaps from alleged sexual exploits of Hollywood actor Errol Flynn.
in loco parentis: 1710, from L., lit. "in the place of a parent."
in medias res: 1786, from L., lit. "in the midst of things."
in memoriam: Latin, lit. “in memory of” (see memory).
in situ: Latin, lit. “in its (original) place or position” (see situate).
in toto: Latin, “as a whole, completely” (see total).
in utero: Latin, lit. “in the uterus” (see uterus).
in vitro: Latin; “in a test tube, culture dish, etc.;” lit. "in glass" (see vitreous).
in vivo: Latin; “within a living organism.”
in't: archaic; 17c. as short for in it.
in- (1): prefix meaning "not, opposite of, without" (also im-/il-/ir- by assimilation of -n- with following consonant), from L. in- "not," cognate with Gk. an-, O.E. un- (see un- (1)).
in- (2): prefix meaning "into, in, on, upon" (also im-; il-; ir- by assimilation of -n- with following consonant), from L. in- "in" (see in). In O.Fr. this often became en-, which usually was respelled in English to conform with Latin, but not always, which accounts for pairs like enquire/inquire. There was a native form, which in W.Saxon usually appeared as on- (cf. O.E. onliehtan "to enlighten"), and some verbs survived into M.E. (cf. inwrite "to inscribe"), but all now seem to be extinct.
in-fighting: 1812, from pugilism, the practice of getting at close quarters with an opponent.
in-joke: 1964, from in + joke.
in-law: 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 phrase is in brother-in-law (13c.); the law is Canon Law, which defines degrees of relationship within which marriage is prohibited.
in-store: also instore, 1961, from in + store. In M.E., instore was a verb meaning "to restore, renew," from L. instaurare.
inability: 15c., from in- (1) + ability.
inable: see enable.
inaccessible: 1550s, from Fr. inaccessible (14c.), from L.L. inaccessibilis, from in- “not” (see in- (1)) + accessibilis, from L. accessus (see accessible). Related: Inaccessibility.
inaccuracy: 1737, “quality or condition of being inaccurate,” from inaccurate + -cy. As an example of this, by 1883.
inaccurate: 1738, from in- (1) “not” + accurate. Related: Inaccurately.
inaction: 1707, from in- (1) + action.
inactive: 1725, from in- (1) + active.
inactivity: 1640s, from in- (1) + activity. Phrase masterly inactivity attested by 1791.
inadequacy: 1787, from inadequate + -cy. Related: Inadequacies.
inadequate: 1670s; see in- (1) + adequate. Related: Inadequately.
inadmissible: 1776, from in- (1) + admissible. Related: Inadmissibility.
inadvertence: 1560s, from Scholastic L. inadvertentia, from in- “not” (see in- (1)) + advertentia, from L. advertere "to direct one's attention to," lit. "to turn toward" (see advertise).
inadvertent: 1650s, ultimately from inadvertence.
inadvertently: 1670s, lit. "inattentively," hence "unintentionally;" see inadvertent + -ly (2).
inadvisable: 1864 (implied in inadvisability), from in- (1) + advisable (see advise).
inalienable: 1640s, from in- (1) + alienable (see alienate). Related: Inalienability.
inamorata: "female lover," 1650s, from It. innamorata, fem. of innamorato, pp. of innamorare "to fall in love," from in "in" + amore "love."
inamorato: masc. of inamorata (q.v.).
inane: "silly, empty-headed," 1819, earlier "empty" (1660s), a back-formation of inanity "emptiness, hollowness" (c.1600), later "silliness" (1753), from Fr. inanité, from L. inanitatem "emptiness," from inanis "empty, void, worthless, useless."
inanimate (adj.): 1570s, from L.L. inanimatus “lifeless,” from in- “not” (see in- (1)) + animatus (see animation). The same word in 17c. also was a verb meaning “to infuse with life,” from the other in- (see in- (2)).
inanition: c.1400, from O.Fr. inanition, from L.L. inanitionem (nom. inanitio), from inanitus, pp. of inanire, from inanis "empty, void, worthless, useless."
inanity: c.1600, literal and figurative, from L. inanitas “emptiness,” noun of quality from inanis (see inane).
inapplicable: 1650s, from in- (1) + applicable. Related: Inapplicability.
inapposite: 1620s, from in- (1) + apposite.
inappropriate: 1804, from in- (1) + appropriate (adj.). Related: Inappropriately.
inapt: 1744, from in- (1) + apt.
inaptitude: 1610s, from in- (1) + aptitude.
inarticulate: c.1600, "not clear or intelligible" (of speech), from L.L. inarticulatus "indistinct," from in- "not" + articulatus, pp. of articulare (see articulation).
inasmuch: contraction of in as much, first attested c.1300 as in als mikel, a Northern form. First contracted to in asmuch, then, beginning 14c. and especially since 17c., to one word.
inattention: 1710, from in- (1) + attention.
inattentive: 1741, from in- (1) + attentive. Related: Inattentiveness.
inaudible: c.1600, from L. inaudibilis, from in- “not” (see in- (1)) + audibilis (see audible). Related: Inaudibly.