lynch (v.) Look up lynch at Dictionary.com
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. The surname is perhaps from Irish Loingseach "sailor."

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. Compare 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]
Also Jedburgh justice (1706). Related: Lynched; lynching.