Source : Webster's Revised Unabridged Dictionary (1913)

Thwarter \Thwart"er\, n. (Far.)
   A disease in sheep, indicated by shaking, trembling, or
   convulsive motions.

Source : WordNet®

thwarter
     n : someone who systematically obstructs some action that others
         want to take [syn: {obstructionist}, {obstructor}, {obstructer},
          {resister}]