The lowest sill, block, or timber supporting a building, located at or below ground level.
The lowest sill of a structure, placed on the foundation or directly on the ground.
"The Pre-Historic Era represented the mudsill of human development."
Origin of mudsill
"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)