- The lowest sill supporting a building, resting directly on the foundation.
- A horizontal timber resting on the ground or on the bottom of a body of water, as to support a scaffold, dam, or bridge pier.
"1685, 'lowest sill of a house,' from mud + sill. The word entered U.S. political history in a speech by James M. Hammond of South Carolina, March 4, 1858, in U.S. Senate, alluding to the very mudsills of society, and the term subsequently was embraced by Northern workers in the pre-Civil War sectional rivalry." (OED, 2007)