The Collaborative International Dictionary of English v.0.48:

Mudsill \Mud"sill`\, n.
   1. The lowest sill of a structure, usually embedded in the
      soil; the lowest timber of a house; also, that sill or
      timber of a bridge which is laid at the bottom of the
      water. See Sill.
      [1913 Webster]

   2. Fig.: A person of the lowest stratum of society; -- a term
      of opprobrium or contempt. [Southern U. S.]
      [Webster 1913 Suppl.]