What is the X in X-mas?
late 15c., from Middle French intérieur and directly from Latin interior "inner, interior, middle," comparative adjective of inter "within" (see inter-). Meaning "of the interior parts of a country" is from 1777; meaning "internal affairs of a country or state" (as in U.S. Department of the Interior) is from 1838. Interior decoration first attested 1807. Interior design from 1927.
"part of a country distant from the coast," 1796, from interior (adj.); meaning "inside of a building or room" is from 1829.