The Collaborative International Dictionary of English v.0.48:

Newcastle \New"cast`le\, prop. n.
   A town in England.
   [PJC]

   Carry coals to Newcastle to do something utterly
      superfluous; to do something useless or wasteful; -- from
      the nearness of Newcastle to the coal-mining district.
      [PJC]

WordNet (r) 3.0 (2006):

Newcastle
    n 1: a port city in northeastern England on the River Tyne; a
         center for coal exports (giving rise to the expression
         `carry coals to Newcastle' meaning to do something
         unnecessary) [syn: Newcastle, Newcastle-upon-Tyne]