The Collaborative International Dictionary of English v.0.48:

Penthouse \Pent"house`\, n. [A corruption of pentice.]
   A shed or roof sloping from the main wall or building, as
   over a door or window; a lean-to. Also figuratively. "The
   penthouse of his eyes." --Sir W. Scott.
   [1913 Webster]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Penthouse \Pent"house`\, a.
   Leaning; overhanging. "Penthouse lid." --Shak. "My penthouse
   eyebrows." --Dryden.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lean-to \Lean"-to`\, n.
   1. (Arch.) A shed or slight building placed against the wall
      of a larger structure and having a single-pitched roof; --
      called also penthouse, and to-fall.
      [1913 Webster]

            The outer circuit was covered as a lean-to, all
            round this inner apartment.           --De Foe.
      [1913 Webster]

   2. A crude, usually temporary shelter comprising a lean-to
      roof braced against any convenient support, as a wall, a
      tree or a pole. The roof may extend all the way to the
      ground.
      [PJC]

WordNet (r) 3.0 (2006):

penthouse
    n 1: an apartment located on the top floors of a building