The Collaborative International Dictionary of English v.0.48:

Billingsgate \Bil"lings*gate`\, n.
   1. A market near the Billings gate in London, celebrated for
      fish and foul language.
      [1913 Webster]

   2. Coarsely abusive, foul, or profane language; vituperation;
      ribaldry.
      [1913 Webster]

WordNet (r) 3.0 (2006):

billingsgate
    n 1: foul-mouthed or obscene abuse [syn: scurrility,
         billingsgate]

The Devil's Dictionary (1881-1906):

BILLINGSGATE, n.  The invective of an opponent.