This is the best etymology I've seen in months. Wikipedia on "filibuster":
The term comes from the early 17th century, where buccaneers were known in England as filibusters. This term had evolved from the Spanish "filibustero" which had come from the French word "flibustier," which itself evolved from the Dutch "vrijbuiter" (freebooter).
That brings up all sorts of other interesting words, of course. I had heard "vrijbuiter" before, but I don't think I had ever previously looked up what it meant; turns out it essentially refers to "free booty." (Note that English "booty" in the sense of "plunder" comes ultimately from Middle Low German "bute," meaning "exchange"; while "booty" in the sense of "ass" comes from Early Modern English "bottie," meaning "buttocks." I think of the latter "booty" as a very modern word, but it dates back in printed English to at least 1928.)
I could have sworn I had talked about "buccaneer" before, but apparently not. It's from French "boucanier," "woodsman"; Wikipedia's entry on buccaneer says that "Boucaniers originally were hunters who were poaching cattle and pigs. They would smoke the meat on wooden frames, 'boucans', so that it could be saved for a later time." (It also notes that the Boucaniers learned to do this from local Arawak, who called it "barbicoa," whence "barbecue.")
"Corsair," another word for pirate, derives eventually from Latin "cursus," meaning "course"; someone who travels along a course? Not sure. And another pirate word, "picaroon"--don't think I've encountered that one before--derives from Spanish "pícaro," meaning "rogue."
And I almost forgot to mention that "pirate" itself comes eventually from Greek "peira," to attempt; interestingly, it's distantly cognate with "fear."