Английский Этимологический словарь - filibuster (n.)

- 1587 as flibutor "pirate," probably ultimately from Du. vrijbuiter "freebooter," used of pirates in the West Indies as Sp. filibustero and Fr. flibustier, either or both of which gave the word to Amer.Eng. Used 1850s and '60s of lawless adventurers from the U.S. who tried to overthrow Central American countries. The legislative sense is c.1851, probably because obstructionist legislators hijacked debate. Not technically restricted to U.S. Senate, but that's where the strategy works best.