IDIOMS

fair game

(idiomatic) An acceptable subject of criticism, scrutiny, or mockery.

Example:   Pretending to be slow is fair game. Pretending to be injured is not.
  The referee ruled the unprecedented play fair game.
  After the middle sister's call from a friend's house, her slice of cake was fair game.
  Anyone running for office is fair game for criticism.
Used other than as an idiom: see fair,‎ game.: a game that is fair, that does not involve cheating, etc.