be open season (on someone or something)

1. To be a period of unrestricted hunting on a particular type of animal. Because of the problems with overpopulation, the governor declared that it's now open season on deer across the entire state. Animal rights activists are not going to be pleased when they hear it's open season on various animals right now. A: "Where did all these pheasants come from? They've been driving me crazy!" B: "Well, you'll be happy to know that it's open season now."

2. By extension, to be a situation or period of time in which someone or something is open to constant, unyielding criticism, scorn, or mistreatment. It's going to be open season if news of this scandal reaches the public. Ever since the despot came to power, it's been open season on any who oppose him. Based on that biting statement, it must be open season on all politicians who disagree with the president's policies.