“beat a dead horse”并非“打一匹死马”的意思，而是“ ‍多此一举，白费口舌”的含义。

【例句】
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。

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！