1835, from earlier Lynch law (1811), likely named after William Lynch (1742-1820) of Pittsylvania, Va., who c.1780 led a vigilance committee to keep order there during the Revolution. Other sources trace the name to Charles Lynch (1736-96) a Virginia magistrate who fined and imprisoned Tories in his district c.1782, but the connection to him is less likely. Originally any sort of summary justice, especially by flogging; narrowing of focus to "extralegal execution by hanging" is 20c. Lynch mob is attested from 1838. The surname is either from O.E. hlinc "hill" or Ir. Loingseach "sailor."
i wonder how this word connoting group vigilantism morphed into a word meaning ritual killings of black men.