Source : Webster's Revised Unabridged Dictionary (1913)

Lynch law \Lynch" law`\
   The act or practice by private persons of inflicting
   punishment for crimes or offenses, without due process of
   law.

   Note: The term Lynch law is said to be derived from a
         Virginian named Lynch, who took the law into his own
         hands. But the origin of the term is very doubtful.

Source : WordNet®

lynch law
     n : the practice of punishing people by hanging without due
         process of law