DuckCorp Dico

Found one definition

1.                 From The Collaborative International Dictionary of English v.0.48:
                   
   
   
                   
                     
   dunno \dun*no"\ (d[u^]n*n[=o]"), phrase [Spelled as often
      pronounced in rapid or slurred speech.]
      A slang shortening of I don't know or don't know; as, dunno
      where I lost my keys; Where'd he go? I dunno..
      [PJC]