He must have been quite a character, William Walker. According to the 1898 Dictionary of Phrase and Fable, Walker was the most notorious “filibuster”. After his 1853 invasion of Lower California failed, another filibustering expedition was more successful. He captured Granada and for a brief time was the self-proclaimed president of Nicaragua.
This use of the word filibuster derives directly from its roots: from Spanish filibustero, a person who plunders, especially a pirate, from French flibustier, from Dutch vrijbuiter, pirate (American Heritage Dictionary).
The more common modern usage of filibuster, as a noun, is lengthy speech given for dilatory purposes to disrupt the functioning of a legislative body. A filibusterer is a person who uses such tactics. The late Senator Wayne Morse comes to mind as an example.
But to the delight of the etymologist in all of us, there now comes the serendipitous conjunction of both usages: former Klansman and current Senator Robert Byrd. The senator is the leading proponent of using the filibuster, or threat thereof, to prevent the United States Senate from performing its constitutional duty of confirming or rejecting nominees to federal judgeships. This comes after a long career as a Walkerian filibuster, seizing the fruits of the labors of others and using it as he sees fit.
Move over William Walker. Despite your prodigious efforts, you are most notorious no longer.