The Collaborative International Dictionary
to-fall
Lean-to \Lean"-to`\, n.
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(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.
The outer circuit was covered as a lean-to, all round this inner apartment.
--De Foe. 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.
Wiktionary
to-fall
n. (alternative form of tofall English)