1a flat on the top floor of a tall building, typically one that is luxuriously fitted.
2 archaic an outhouse or shelter with a sloping roof, built on to the side of a building.
Middle Englishpentis(in sense 2), shortening of Old French apentis, based on late Latin appendicium 'appendage', from Latin appendere 'hang on'. The change of form in the 16th century was by association with French pente 'slope' and house
penthouse in other Oxford dictionaries
Definition of penthouse in the US English dictionary