The filibuster may be unconstitutional, creating as it does a mechanism for a Senatorial veto.
One thing's for sure: Filibusters are also pre-constitutional. They are even pre-colonial.
The original filibusters were pirates of the Caribbean, and the contemporary Senate procedure continues to bear traces of the word's origins in the disruptive and lawless practices of the privateers who boosted the goods of ships traveling under Spanish sail. Later, the word came to describe an American movement with a base in the pre-Civil War South to seize Spanish West Indian and Central American lands and goods in the name of Manifest Destiny.
The word derives from a Dutch term for pirate and began to be applied to efforts "to hold the Senate floor in order to prevent a vote on a bill" in the in the 1850s, according to the Senate Historical Office. It is believed to derive from the Dutch word vrijbuiter, which means "to plunder," with vrij meaning "free" and buit meaning "booty."
And booty, according to Michael Sheen, a retired Chicago English teacher who writes the WordMall blog, means "collective plunder or spoils" such as "household goods seized and carried off" by armies.
These "freebooters" or "filibusters" traveled under no nation's sail -- they used various iterations of the Jolly Roger, instead -- and often sought to "privateer" goods and gold being transported by Spanish ships in the Caribbean, from slaves to gold. The word to describe them was first recorded in English in the late 16th century, according to the Oxford English Dictionary.