below the belt – an unfair attack (in boxing that is not allowed)

腰带下方 - 不正当的攻击（在拳击比赛中这是不允许的）

例句：

There was no need to mention my personal problems to the press. That was below the belt.

我没必要跟媒体谈论我的私人问题。这是不合理的。