Penthouse
Middle English pentis (in penthouse (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.
wiktionary
From Anglo-Norman pentiz(“ pentice”), from apendiz(“ appentice”), ultimately from a suffixed form of Latin appendō(“I append”). Altered by folk etymology to appear to be a compound of house. Doublet of appentice and pentice.
etymonline
penthouse (n.)
c. 1300, pentis, pendize, "a shed or sloping roof projecting from a main wall or the side or end of a building," from Anglo-French pentiz, a shortening of Old French apentis "attached building, appendage," from Medieval Latin appendicium, from Latin appendere "to hang" (see append).
The modern spelling is from c. 1530 by folk etymology influence of 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 the stable where Jesus was born as a "penthouse"); meaning "apartment or small house built on the roof of a skyscraper" is attested by 1921, from which time dates its association with luxury.
