• WordNet 3.6
    • n upsetter an unexpected winner; someone who defeats the favorite competitor
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Century Dictionary and Cyclopedia
    • n upsetter One who or that which upsets; also, one who or that which sets up; specifically, a tool used in upsetting a tire.
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In literature:

He had always been an upsetter of conventions, and a law unto himself.
"The Divine Fire" by May Sinclair
It was at this happy pitch that he replied to the prime upsetter of his peace.
"The Outcry" by Henry James
He argued as modern men do who think the hero, the upsetter of ordinary calculations, is gone from us.
"The Ordeal of Richard Feverel" by George Meredith

In news:

The Upsetter is a pied piper for weirdness, that rare artist whose obvious madness exists in perfect harmony with his uncanny ability.
Taqarub plays upsetter in the Maryland Sprint Handicap.