penthouse: [14] Penthouse has no etymological connection with house. It comes from Anglo- Norman *pentis, an abbreviated version of Old French apentis. This in turn went back to Latin appendicium ‘additional attached part’, a derivative of appendēre ‘attach’ (source of English append [15] and appendix [16]), which was a compound verb formed from the prefix ad- ‘to’ and pendēre ‘hang’ (source of English pending, pendulum, etc).
It arrived in English as pentis, and was used for a sort of ‘lean-to with a sloping roof’. A perceived semantic connection with houses led by the late 14th century to its reformulation as penthouse, but its application to a ‘(luxurious) flat on top of a tall block’ did not emerge until the 20th century. => append, pendulum
penthouse (n.)
pendize, early 14c., from Anglo-French pentiz, a shortening of Old French apentis "attached building, appendage," from Medieval Latin appendicium, from Latin appendere "to hang" (see append). Modern spelling is from c. 1530, by folk etymology influence of Middle French pente "slope," and English house (the meaning at that time was "attached building with a sloping roof or awning"). Originally a simple structure (Middle English homilies describe Jesus' birthplace in the manger as a "penthouse"); meaning "apartment or small house built on the roof of a skyscraper" first recorded 1921, from which time dates its association with luxury.