The Collaborative International Dictionary of English v.0.48:

Lynch \Lynch\, v. t. [imp. & p. p. Lynched; p. pr. & vb. n.
   Lynching.] [See Note under Lynch law.]
   To inflict punishment upon, especially death, without the
   forms of law, as when a mob captures and hangs a suspected
   person. See Lynch law.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lynch
    v 1: kill without legal sanction; "The blood-thirsty mob lynched
         the alleged killer of the child"