Sense of "leniency, forbearance of restraint or control of another, gratification of desire or humor" is attested from late 14c. That of "yielding to one's inclinations" (technically self-indulgence) in English is from 1630s. In British history, Indulgence also refers to grants of certain liberties to Nonconformists under Charles II and James II, as special favors rather than legal rights. The sale of indulgences in the original Church sense was done at times merely to raise money and was widely considered corrupt; the one in 1517 helped to spark the Protestant revolt in Germany.
in the sports sense, 1879, originally in cricket, "taking three wickets on three consecutive deliveries;" extended to other sports c. 1909, especially ice hockey ("In an earlier contest we had handed Army a 6-2 defeat at West Point as Billy Sloane performed hockey's spectacular 'hat trick' by scoring three goals" ["Princeton Alumni Weekly," Feb. 10, 1941]). So called allegedly because it entitled the bowler to receive a hat from his club commemorating the feat (or entitled him to pass the hat for a cash collection), but the term probably has been influenced by the image of a conjurer pulling objects from his hat (an act attested by 1876). The term was used earlier for a different sort of magic trick:
Place a glass of liquor on the table, put a hat over it, and say, "I will engage to drink every drop of that liquor, and yet I'll not touch the hat." You then get under the table; and after giving three knocks, you make a noise with your mouth, as if you were swallowing the liquor. Then, getting from under the table, say "Now, gentlemen, be pleased to look." Some one, eager to see if you have drunk the liquor, will raise the hat; when you instantly take the glass and swallow the contents, saying, "Gentlemen I have fulfilled my promise: you are all witnesses that I did not touch the hat." ["Wit and Wisdom," London, 1860]
1580s, flibutor "pirate," especially, in history, "West Indian buccaneer of the 17th century" (mainly French, Dutch, and English adventurers), probably ultimately from Dutch vrijbueter (now vrijbuiter) "freebooter," a word which was used of pirates in the West Indies in Spanish (filibustero) and French (flibustier, earlier fribustier) forms. See freebooter.
According to Century Dictionary, the spread of the word is owing to a Dutch work ("De Americaensche Zee-Roovers," 1678) "written by a bucaneer named John Oexmelin, otherwise Exquemelin or Esquemeling, and translated into French and Spanish, and subsequently into English (1684)." Spanish inserted the -i- in the first syllable; French is responsible for the -s-, inserted but not originally pronounced, "a common fact in 17th century F[rench], after the analogy of words in which an original s was retained in spelling, though it had become silent in pronunciation" [Century Dictionary].
In American English, from 1851 in reference to lawless military adventurers from the U.S. who tried to overthrow Central American governments. The major expeditions were those of Narciso Lopez of New Orleans against Cuba (1850-51) and by William Walker of California against the Mexican state of Sonora (1853-54) and against Nicaragua (1855-58).
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 noun in the legislative sense is not in Bartlett (1859) and seems not to have been in use in U.S. legislative writing before 1865 (filibustering in this sense is from 1861). Probably the extension in sense is because obstructionist legislators "pirated" debate or overthrew the usual order of authority. Originally of the senator who led it; the maneuver itself so called by 1893. Not technically restricted to U.S. Senate, but that's where the strategy works best. [The 1853 use of filibustering by U.S. Rep. Albert G. Brown of Mississippi reported in the Congressional Globe and cited in the OED does not refer to legislative obstruction, but to national policy toward Cuba.]