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.
"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.