from
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]