• WordNet 3.6
    • n thwarter someone who systematically obstructs some action that others want to take
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Webster's Revised Unabridged Dictionary
    • n Thwarter (Far) A disease in sheep, indicated by shaking, trembling, or convulsive motions.
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Century Dictionary and Cyclopedia
    • n thwarter One who crosses or goes athwart.
    • n thwarter One who or that which thwarts or crosses.
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In literature:

When I entered who should I see but the eternal thwarter of my happiness, the ever-present Nicholson!
"Wild Western Scenes" by John Beauchamp Jones