DuckCorp Dico

Found one definition

1.                 From The Collaborative International Dictionary of English v.0.48:
                   
   
   
                   
                     
   XP \XP\ [Belongs here in appearance only.]
      The first two letters of the Greek word XRISTOS, Christ; --
      an abbreviation used with the letters separate or, oftener,
      in a monogram, often inclosed in a circle, as a symbol or
      emblem of Christ. It use as an emblem was introduced by
      Constantine the Great, whence it is known as the
      Constantinian symbol, or
      monogram. See Labarum.
         [Webster 1913 Suppl.]