The story
"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)
Etymology from Wiktionary, CC BY-SA 4.0.