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Webster 1913 Edition

Adj.

(Arch.)

Having only one slope or pitch; – said of a roof.

Noun.

(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.

lean-to ‎(plural lean-tos)

A shelter with a sloped roof; also a building with a similar construction attached to the side of a building as an extension.
- Mark Twain
  Now we was in the lean-to, and heard trampings close by outside. So we crept to the door, and Tom stopped us there and put his eye to the crack, but couldn't make out nothing, it was so dark […]
- 1913, Joseph C. Lincoln, chapter 8, in Mr. Pratt's Patients:
  Philander went into the next room, which was just a lean-to hitched on to the end of the shanty, and came back with a salt mackerel that dripped brine like a rainstorm. Then he put the coffee pot on the stove and rummaged out a loaf of dry bread and some hardtack.

type of shelter