The Collaborative International Dictionary of English v.0.48:

Curbstone \Curb"stone`\ (k?rb"st?n`), n.
   A stone set along a margin as a limit and protection, as
   along the edge of a sidewalk next the roadway; an edge stone.
   [1913 Webster]

   Curbstone broker. See under Broker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

curbstone
    n 1: a paving stone forming part of a curb [syn: curbstone,
         kerbstone]