A person or thing that causes disruption or disturbance. The unexpected resignation of the CEO was a major upsetter for the company.
A competitor or team that unexpectedly defeats a favored opponent. The underdog team was the upsetter of the tournament, defeating the top-seeded team in the finals.
A forging machine or hammer used to increase the diameter of a heated metal piece by compressing it lengthwise. The shop's new upsetter allowed them to forge thicker bolt heads more efficiently.