DuckCorp Dico

Found one definition

1.                 From The Devil's Dictionary (1881-1906):
                   
   
   
                   
                     
   FLAG, n.  A colored rag borne above troops and hoisted on forts and
   ships.  It appears to serve the same purpose as certain signs that one
   sees and vacant lots in London -- "Rubbish may be shot here."