1680s, "foundation beam of a dam, railroad, house, or other structure," from mud + sill. The word entered U.S. political history in a figurative sense in a speech by James M. Hammond (1807-1864) of South Carolina, March 4, 1858, in the U.S. Senate, alluding to the necessary lowest class of society. In the speech he also took the ground that Northern white laborers were slaves in fact, if not in name. The term subsequently was embraced by Northern workers in the pre-Civil War sectional rivalry.
In all social systems there must be a class to do the menial duties, to perform the drudgery of life. This is a class requiring but a low order of intellect and but little skill. Its requisites are vigor, docility, fidelity. Such a class you must have, or you would not have that other class which leads progress, civilization, and refinement. It constitutes the very mud-sill of society and of political government; and you might as well attempt to build a house in the air as to build either the one or the other except on this mud-sill. [Hammond]