below the belt意思是“不合乎规则，不公正，不符合道德”。

【例句】
I do not believe in fighting below the belt. The dignity of a fight is lost when people do that.
我不相信不公正的打架。当人们这样做的时候，战斗的尊严就丧失了。

The shot that he took at him was completely below the belt. Decent people do not compete like this.
他向他开的那一枪完全是不正当的。正派的人不会这样竞争的。