noun

• (dated) Piracy, freebooting; the waging of unauthorised war.
• (US) The practice of forcefully and unauthorisedly acquiring control of foreign land.
  "All this was simply weakness; but in turning from the conduct of the Finances by the administration, to consider its management of Filibusterism, we pass from the consideration of acts of mere debility to the consideration of acts which have a color of duplicity in them. On the Filibusters, as on the Finances, the First Annual Message of the President was outspoken and forcible. It characterized the past and proposed doings of William Walker and his crew, as the common sense and common conscience of the world had already characterised them, as nothing short of piracy and murder."