The Collaborative International Dictionary of English v.0.48:

Ironsides \I"ron*sides"\, n.
   A cuirassier or cuirassiers; also, hardy veteran soldiers; --
   applied specifically to Cromwell's cavalry.

   Old Ironsides. (U. S. Hist.) A nickname for the U.S.S.
      Constitution, a sailing ship which fought in the American
      Revolutionary war, and now functions as a floating museum
      in Boston harbor. It was given its nickname because
      cannonballs bounced off its hard wooden sides.
      [PJC]

WordNet (r) 3.0 (2006):

Old Ironsides
    n 1: a United States 44-gun frigate that was one of the first
         three naval ships built by the United States; it won
         brilliant victories over British frigates during the War of
         1812 and is without doubt the most famous ship in the
         history of the United States Navy; it has been rebuilt and
         is anchored in the Charlestown Navy Yard in Boston [syn:
         Constitution, Old Ironsides]