whitecapper

white +‎ cap +‎ -er, after the white hoods worn by some.

whitecapper (plural whitecappers)

1. (US, historical) Synonym of whitecap (“type of vigilante”)
   • 1905 April 12, “Mexican Laborers Warned to Leave: Whitecappers Posted Warnings on Doors of Cabins Occupied by the Mexicans Near Horatio”, in Arkansas Gazette‎, Little Rock, Arkansas, via Newspapers.com, page 1:

     Deputy United States Marshal H. B. Holman returned here on the Kansas City Southern Railroad, to which place he was called by a message from the manager of a large fruit-growing company, asking protection from whitecappers.

   • 1993, William Fitzhugh Brundage, Lynching in the New South: Georgia and Virginia, 1880–1930‎, Urbana, Illinois: University of Illinois Press, →ISBN, page 27:

     To the gratification of many local whites, an all-white jury acquitted Brown of the charge of murder and ruled that the shooting of the whitecapper had been a justifiable homicide.