curveball
noun /ˈkɜːvbɔːl/
  /ˈkɜːrvbɔːl/
(also curve)
- (in baseball) a ball that moves in a curve when it is thrown to the batter- His curveball has lost its bite.
 
- (North American English, informal) something that is unexpected and difficult to deal with- On of the journalists threw the senator a curveball (= surprised him/her by asking a difficult question).
- The plot throws a few curveballs along the way to keep you guessing.