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, but to national policy toward Cuba.]
1560s, loan-translation of Dutch vrijbuiter "plunderer, robber," from vrijbuiten "to rob, plunder," from vrijbuit "plunder," literally "free booty," from vrij "free" (from Proto-Germanic *frijaz, from PIE root *pri- "to love") + buit "booty," from buiten "to exchange or plunder," from Middle Dutch buten, related to Middle Low German bute "exchange" (see booty).
The English word, Danish fribytter, Swedish fribytare, and German Freibeuter were formed on the model of the Dutch word, which is the source of filibuster (q.v.). The back-formed verb freeboot is recorded from 1590s. Related: Freebooting; freebootery.
prī-, Proto-Indo-European root meaning "to love." In some languages (notably Germanic and Celtic) it developed derivatives with the sense "free, not in bondage," perhaps via "beloved" or "friend" being applied to the free members of one's clan (as opposed to slaves).
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit priyah "own, dear, beloved," priyate "loves;" Old Church Slavonic prijati "to help," prijatelji "friend;" Welsh rhydd "free;" Old English freo "exempt from; not in bondage, acting of one's own will," Gothic frijon "to love," Old English freod "affection, friendship, peace," friga "love," friðu "peace," Old Norse Frigg, name of the wife of Odin, literally "beloved" or "loving."