Etymology
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internal (adj.)
early 15c., "extending toward the interior," from Medieval Latin internalis, from Latin internus "within, inward, internal," figuratively "domestic," expanded from pre-Latin *interos, *interus "on the inside, inward," from PIE *en-ter- (source also of Old Church Slavonic anter, Sanskrit antar "within, between," Old High German unter "between," and the "down" sense of Old English under); suffixed (comparative) form of root *en "in."

Meaning "situated within" is from 1590s. Meaning "of or pertaining to the domestic affairs of a country (as in internal revenue) is from 1795; the notion is "pertaining to the subject itself; independent of others." Internal-combustion in reference to an engine in which fuel is burned inside it, is from 1884. Related: Internally.
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internalize (v.)
1856, American English, from internal + -ize. Related: Internalized; internalizing.
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intern (v.1)

1866, "to confine within set limits," from French interner "send to the interior, confine," from interne "inner, internal" (14c.), from Latin internus "within, internal" (see internal; also compare intern (n.)).

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intern (n.)
"one working under supervision as part of professional training," originally "assistant resident physician, doctor in training in a hospital," 1879, American English, from French interne "assistant doctor," noun use of interne "internal," from Latin internus "within, inward" (see internal). Extended to other professions (originally teaching) from 1963 in reference to one under training and acquiring practical experience.
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*en 
Proto-Indo-European root meaning "in."

It forms all or part of: and; atoll; dysentery; embargo; embarrass; embryo; empire; employ; en- (1) "in; into;" en- (2) "near, at, in, on, within;" enclave; endo-; enema; engine; enoptomancy; enter; enteric; enteritis; entero-; entice; ento-; entrails; envoy; envy; episode; esoteric; imbroglio; immolate; immure; impede; impend; impetus; important; impostor; impresario; impromptu; in; in- (2) "into, in, on, upon;" inchoate; incite; increase; inculcate; incumbent; industry; indigence; inflict; ingenuous; ingest; inly; inmost; inn; innate; inner; innuendo; inoculate; insignia; instant; intaglio; inter-; interim; interior; intern; internal; intestine; intimate (adj.) "closely acquainted, very familiar;" intra-; intricate; intrinsic; intro-; introduce; introduction; introit; introspect; invert; mesentery.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit antara- "interior;" Greek en "in," eis "into," endon "within;" Latin in "in, into," intro "inward," intra "inside, within;" Old Irish in, Welsh yn, Old Church Slavonic on-, Old English in "in, into," inne "within, inside."
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IRS 
also I.R.S., initialism (acronym) of Internal Revenue Service, U.S. federal government tax collection agency, attested by 1954. The office dates to 1862; name changed 1953 from Bureau of Internal Revenue.
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interior (n.)
"part of a country distant from the coast," 1796, from interior (adj.); meaning "internal part, inside" is from 1828. Meaning "internal affairs of a country or state" (as in U.S. Department of the Interior) is from 1826. The Latin adjective also was used as a noun.
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