Etymology

Noun

1. A freebooter, or mercenary soldier.

   1890 These duties involved prodigious physical and mental exertion, in a climate deadly to Europeans. They also involved much voyaging in waters haunted by filibusters and buccaneers. But nothing appears to daunt Labat. As for the filibusters, he becomes their comrade and personal friend; -- he even becomes their chaplain, and does not scruple to make excursions with them. — Lafcadio Hearn, Two Years in the French West Indies.

2. Delaying tactics, especially long, often irrelevant speeches given in order to delay progress or the making of a decision, especially on the floor of the US Senate.

   1919 But as the case had dragged on interminably, and he believed, and the world believed, and the Canadians themselves knew, that they intended to filibuster and postpone as long as possible, he took the common-sense way to a settlement. — William Roscoe Thayer, Theodore Roosevelt: An Intimate Biography, Chapter 11.

3. A member of a legislative body causing such obstruction.

Verb

1. To take part in a private military action in a foreign country.
2. To use obstructionist tactics in a legislative body.