The Collaborative International Dictionary of English v.0.48:

drop cloth \drop" cloth`\ n.
   a sheet of material used to cover objects or surfaces while
   painting a ceiling or wall of a house, so as to protect
   objects from being marred by drops of paint splashed
   inadvertantly in the painting process. Originally such drop
   cloths were made of cloth, but more recently paper or plastic
   have also commonly been used.
   [PJC]

WordNet (r) 3.0 (2006):

drop cloth
    n 1: a curtain that can be lowered and raised onto a stage from
         the flies; often used as background scenery [syn: drop
         curtain, drop cloth, drop]
    2: a large piece of cloth laid over the floor or furniture while
       a room is being painted