Etymology

Noun

1. An outhouse or other structure (especially one with a sloping roof) attached to the outside wall of a building.
   • 1826: William Eusebius Andrews, Review of Fox's Book of Martyrs, WE Andrews, pp. 386-7:

     At length, recommending himself to God, he let go one end of his cord, and suffered himself to fall down upon an old shed or penthouse, which, with the weight of his body, fell in with great noise.

2. An apartment or suite on the top floor of a tall building, especially as associated with being expensive or luxurious.
   • 1995: Mary Ellen Waithe, Contemporary Women Philosophers: 1900-Today, Springer, p. 214:

     Night of January 16th is the story of a woman on trial for pushing her wealthy boss-lover from a Manhattan penthouse.

3. Any of the sloping roofs at the side of a real tennis court.
   • 2005, Tony Collins (ed.), Encyclopaedia of Traditional British Rural Sports, Routledge, page 262,

     An odd derivative of real tennis lasted until the latter part of the eighteenth century at Rattray in Perthshire. It was played in the churchyard by two pairs of men, and the method for starting the play was to throw the ball onto the church roof, using it like the sloping penthouse of the tennis court.