filibuster (n.) Look up filibuster at Dictionary.com
1580s, flibutor "pirate," probably ultimately from Dutch vrijbuiter "freebooter," a word which was used of pirates in the West Indies in Spanish (filibustero) and French (flibustier) forms, either or both of which gave the word to American English (see freebooter). Used in the 1850s and '60s of lawless adventurers from the U.S. who tried to overthrow Central American governments.
FILIBUSTERING is a term lately imported from the Spanish, yet destined, it would seem, to occupy an important place in our vocabulary. In its etymological import it is nearly synonymous with piracy. It is commonly employed, however, to denote an idea peculiar to the modern progress, and which may be defined as the right and practice of private war, or the claim of individuals to engage in foreign hostilities aside from, and even in opposition to the government with which they are in political membership. ["Harper's New Monthly Magazine," January 1853]
The legislative sense is not in Bartlett (1859) and seems not to have been in use in U.S. legislative writing before 1865. Probably the extension in sense is because obstructionist legislators "pirated" debate or overthrew the usual order of authority. Not technically restricted to U.S. Senate, but that's where the strategy works best.
filibuster (v.) Look up filibuster at Dictionary.com
1853 in both the freebooting and the legislative senses, from filibuster (n.). Related: Filibustered; filibustering.