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1835, from earlier Lynch law (1811), likely named after William Lynch (1742-1820) of Pittsylvania, Virginia, who c.1780 led a vigilance committee to keep order there during the Revolution. Other sources trace the name to Charles Lynch (1736-1796) 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 perhaps from Irish Loingseach "sailor." Cf. earlier Lydford law, from a place in Dartmoor, England, "where was held a Stannaries Court of summary jurisdiction" [Weekley], hence:
Lydford law: is to hang men first, and indite them afterwards. [Thomas Blount, "Glossographia," 1656]Related: Lynched; lynching.