1580s, flibutor "pirate," especially, in history, "West Indian buccaneer of the 17th century" (mainly French, Dutch, and English adventurers), probably ultimately from Dutch vrijbueter (now vrijbuiter) "freebooter," a word which was used of pirates in the West Indies in Spanish (filibustero) and French (flibustier, earlier fribustier) forms. See freebooter.
According to Century Dictionary, the spread of the word is owing to a Dutch work ("De Americaensche Zee-Roovers," 1678) "written by a bucaneer named John Oexmelin, otherwise Exquemelin or Esquemeling, and translated into French and Spanish, and subsequently into English (1684)." Spanish inserted the -i- in the first syllable; French is responsible for the -s-, inserted but not originally pronounced, "a common fact in 17th century F[rench], after the analogy of words in which an original s was retained in spelling, though it had become silent in pronunciation" [Century Dictionary].
In American English, from 1851 in reference to lawless military adventurers from the U.S. who tried to overthrow Central American governments. The major expeditions were those of Narciso Lopez of New Orleans against Cuba (1850-51) and by William Walker of California against the Mexican state of Sonora (1853-54) and against Nicaragua (1855-58).
FILIBUSTERING is a term lately imported from the Spanish, yet destined, it would seem, to occupy an important place in our vocabulary. In its etymological import it is nearly synonymous with piracy. It is commonly employed, however, to denote an idea peculiar to the modern progress, and which may be defined as the right and practice of private war, or the claim of individuals to engage in foreign hostilities aside from, and even in opposition to the government with which they are in political membership. [Harper's New Monthly Magazine, January 1853]
The noun in the legislative sense is not in Bartlett (1859) and seems not to have been in use in U.S. legislative writing before 1865 (filibustering in this sense is from 1861). Probably the extension in sense is because obstructionist legislators "pirated" debate or overthrew the usual order of authority. Originally of the senator who led it; the maneuver itself so called by 1893. Not technically restricted to U.S. Senate, but that's where the strategy works best. [The 1853 use of filibustering by U.S. Rep. Albert G. Brown of Mississippi reported in the Congressional Globe and cited in the OED does not refer to legislative obstruction, merely to national policy toward Cuba.]