The Collaborative International Dictionary of English v.0.48:

Mopboard \Mop"board`\, n. (Carp.)
   A narrow board nailed against the wall of a room next to the
   floor; skirting board; baseboard. See Baseboard.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mopboard
    n 1: a molding covering the joint formed by a wall and the floor
         [syn: baseboard, mopboard, skirting board]