The Collaborative International Dictionary of English v.0.48:

Misadventure \Mis`ad*ven"ture\ (?; 135), n. [OE. mesaventure, F.
   m['e]saventure.]
   Mischance; misfortune; ill luck; unlucky accident; ill
   adventure. --Chaucer.
   [1913 Webster]

   Homicide by misadventure (Law), homicide which occurs when
      a man, doing a lawful act, without any intention of
      injury, unfortunately kills another; -- called also
      excusable homicide. See Homicide. --Blackstone.
      [1913 Webster]

   Syn: Mischance; mishap; misfortune; disaster; calamity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

misadventure
    n 1: an instance of misfortune [syn: mishap, misadventure,
         mischance]