The Collaborative International Dictionary of English v.0.48:

Thwarter \Thwart"er\, n. (Far.)
   A disease in sheep, indicated by shaking, trembling, or
   convulsive motions.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thwarter
    n 1: someone who systematically obstructs some action that
         others want to take [syn: obstructionist, obstructor,
         obstructer, resister, thwarter]