The Collaborative International Dictionary of English v.0.48:

Filibuster \Fil"i*bus`ter\, n. [Sp. flibuster, flibustero,
   corrupted fr. E. freebooter. See Freebooter.]
   A lawless military adventurer, especially one in quest of
   plunder; a freebooter; -- originally applied to buccaneers
   infesting the Spanish American coasts, but introduced into
   common English to designate the followers of Lopez in his
   expedition to Cuba in 1851, and those of Walker in his
   expedition to Nicaragua, in 1855.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Filibuster \Fil"i*bus*ter\, v. i. [imp. & p. p. Fillibustered;
   p. pr. & vb. n. Filibustering.]
   1. To act as a filibuster, or military freebooter.
      --Bartlett.
      [1913 Webster]

   2. To delay legislation, by dilatory motions or other
      artifices. [political cant or slang, U.S.] --Bartlett.
      [1913 Webster]

WordNet (r) 3.0 (2006):

filibuster
    n 1: a legislator who gives long speeches in an effort to delay
         or obstruct legislation that he (or she) opposes [syn:
         filibuster, filibusterer]
    2: (law) a tactic for delaying or obstructing legislation by
       making long speeches
    v 1: obstruct deliberately by delaying