Noah Webster
Sill
SILL, noun [Latin solum; allied to solid. The primary sense is probably to lay, set or throw down.]
1. Properly, the basis of foundationof a thing; appropriately, a piece of timber on which a building rests; the lowest timber of any stucture; as the sills of a house, of a bridge, of a loom and the like.
2. The timber or stone at the foot of a door; the threshhold.
3. The timber or stone on which a window frame stands; or the lowest piece in a window frame.
4. The shaft or thill of a carriage. [Local.]