ˌəpˈsed.ə(r) noun
( -s )
: one that upsets: as
a. : a person who causes upsets
the businessman … is the great upsetter on the contemporary scene — M.S.Rukeyser
b. : an upsetting machine
c. : a forging press operator who increases the breadth of a piece of metal by heating and pressing it so that it spreads