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 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" is attested by 1921, from which time dates its association with luxury.