“beat a dead horse”不能翻译成“鞭打一匹死马”，引申意思是多此一举，白费口舌。

【例句】
Why do you insist on beating a dead horseby bringing up these ancient problems of ours?
你为什么旧事重提，这岂不是白费唇舌！

I hate to beat a dead horse, but it is very important that you understand this before you leave.
我不想多此一举，但你离开以前必须了解这个。