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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

EXCUSABLE HOMICIDE, crim. law. The killing of a human being, when the party
killing is not altogether free from blame, but the necessity which renders
it excusable, may be said to be partly induce by his own act. 1 East, P. C.
220.