Etymology
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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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interior (adj.)

late 15c., from Latin interior "inner, interior, middle," comparative adjective of inter "within" (from PIE *enter "between, among," comparative of root *en "in"). Specific meaning "away from the coast, of the interior parts of a country" is from 1777. Interior decoration first attested 1769; interior decorator is from 1830. Interior design from 1927.

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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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in-country (n.)

"interior regions" of a land, 1560s, from in (prep.) + country.

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inboard (adv.)

"within the hull or interior of a ship," 1830, from in (adv.) + board (n.2).

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up-country (n.)

"interior regions," 1680s, from up- + country (n.). As an adjective from 1810; as an adverb from 1864.

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Muscat 

capital of Oman, from Arabic Masqat, said to mean "hidden" (it is isolated from the interior by hills).

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upholstery (n.)

"upholsterer's work, furniture covered with textile materials, interior fittings made from textiles," 1640s; see upholster + -y (4).

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stomatitis (n.)

"inflammation of the interior of the mouth," 1859, from Greek stomato- (before vowels stomat-), combining form from stem of stoma "mouth" (see stoma) + -itis "inflammation."

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