from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The horizontal timber nearest the ground in the frame of a building.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A timber beam used as the foundation for a building.
- n. The lowest beam of a door-frame; the threshold.
from The Century Dictionary and Cyclopedia
- n. The timber of a building which lies next to the ground; the ground-plate; the sill.
- n. In mining, the bottom piece of a wooden gallery-frame.
Apparently from ground + sill. (Wiktionary)