Filibuster.
A piratical adventurer. The most notorious was William Walker, who was shot in 1855. (French, flibustier, a corruption of our “freebooter;” German, freibeuter; Spanish, filibustero; Dutch, vrijbueter.) (See Buccaneer.)
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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.