What’s the connecting between the words filibuster and freebooter?

The answer is, they both come from the same Dutch word vrijbuiter [ˈvrɛi̯bœy̯tər] (plunderer, robber), from vrij (free), buit (booty) and‎ -er (agent suffix).

A freebooter as originally “an adventurer who pillages, plunders or wages ad-hoc war on other nations”, and apparently also means “one who rehosts online media without authorization”. It is a calque translation from Dutch, and was first recorded in English in the 1560s [source].

A filibuster originally meant “a mercenary soldier; specifically, a mercenary who travelled illegally in an organized group from the United States to a country in Central America or the Spanish West Indies in the mid-19th century seeking economic and political benefits through armed force”. Over time it also came to mean, “A tactic (such as giving long, often irrelevant speeches) employed to delay the proceedings of, or the making of a decision by, a legislative body, particularly the United States Senate”.

Filibuster was first recorded in English in the 1580s as flibutor. It was borrowed from the Spanish filibustero (pirate), from French flibustier (pirate), from the Dutch vrijbuiter.

I discovered this from Bill Bryson’s Made In America: An Informal History of American English, which I’m reading at the moment.

Filibustering freebooters! sounds like the kind of curse Captain Haddock uses in the Tintin stories. He does in fact say Filibuster(s)! and Fancy-dress freebooter!, but not Filibustering freebooters!, as far as I can discover.