This idiom or expression like phrase means:

Not fair.
Not by the rules.

A. Did you hear what your opponent said about your background yesterday?
B. Yes. It was below the belt, not worthy of him, and definitely not appreciated!!

This idiom or expression refers to:

A period when certain restrictions don’t apply.

A. You know everybody’s killing rats?
B. I know, it’s open season on them!
A. Speaking of open season, are Republicans still in trouble?
B. Yeah, everybody is still blaming them.

Also, related:
Fair game.

This idiom or expression refers to:

The object of an attack, especially when certain restrictions do not apply.

Celebrities’ personal lives seem to be fair game these days.
The candidates’ financial activities are fair game and open to questions and criticism.

Also, related:
Open season.