vigilante (n.)

"member of a vigilance committee," 1856, American English, from Spanish vigilante, literally "watchman," from Latin vigilantem (nominative vigilans) "watchful, anxious, careful," from vigil "watchful, awake" (from PIE root *weg- "to be strong, be lively"). Vigilant man in same sense is attested from 1824 in a Missouri context. Vigilance committees kept informal rough order on the U.S. frontier or in other places where official authority was imperfect.