The Collaborative International Dictionary of English v.0.48:

Chance-medley \Chance"-med`ley\, n. [Chance + medley.]
   1. (Law) The killing of another in self-defense upon a sudden
      and unpremeditated encounter. See Chaud-Medley.
      [1913 Webster]

   Note: The term has been sometimes applied to any kind of
         homicide by misadventure, or to any accidental killing
         of a person without premeditation or evil intent, but,
         in strictness, is applicable to such killing as happens
         in defending one's self against assault. --Bouvier.
         [1913 Webster]

   2. Luck; chance; accident. --Milton. --Cowper.
      [1913 Webster]

WordNet (r) 3.0 (2006):

chance-medley
    n 1: an unpremeditated killing of a human being in self defense

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

CHANCE-MEDLEY, criminal law. A sudden affray. This word is sometimes applied
to any kind of homicide by misadventure, but in strictness it is applicable
to such killing only as happens se defendendo. (q.v.) 4 Bl. Com. 184.