upsetter

/ʌpˈsɛtər/
noun
  1. A person or thing that causes someone to feel unhappy, worried, or disturbed.
    • Don't be an upsetter — try to keep the peace.
    • He was known as an upsetter because he always started arguments.
    • The loud music was an upsetter for the neighbors trying to sleep.
  2. A person or team that defeats a stronger opponent unexpectedly, especially in sports.
    • She was the upsetter in the race, finishing first despite being a newcomer.
    • The underdog proved to be a true upsetter in the championship.
    • The small school became the upsetter of the tournament by beating the top team.
Antonyms
What does "upsetter" mean? | whatsthatwordmean | whatsthatwordmean