penetralium
/ˌpɛnɪˈtreɪlɪəm/noun
An innermost or most secret part or place; the interior of a building.
Origin
Early 19th century; earliest use found in John Keats (1795–1821), poet. Back-formation from penetralia, on the assumption that classical Latin penetrālia, plural, corresponds to a singular form *penetrālium.