The Collaborative International Dictionary of English v.0.48:

Agitator \Ag"i*ta`tor\, n. [L.]
   1. One who agitates; one who stirs up or excites others; as,
      political reformers and agitators.
      [1913 Webster]

   2. (Eng. Hist.) One of a body of men appointed by the army,
      in Cromwell's time, to look after their interests; --
      called also adjutators. --Clarendon.
      [1913 Webster]

   3. An implement for shaking or mixing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

agitator
    n 1: one who agitates; a political troublemaker [syn:
         agitator, fomenter]

The Devil's Dictionary (1881-1906):

AGITATOR, n.  A statesman who shakes the fruit trees of his neighbors
-- to dislodge the worms.