penthouse noun a small structure, esp. one with a sloping roof, attached to a larger building a sloping roof, or, sometimes, an awning, etc. extending out from a wall or building an apartment or other houselike structure built on the roof of a building a luxury apartment on an upper floor, esp. the top floor of a buildingOrigin of penthousealtered (infl. by house) ; from pentice ; from Middle Engl