bad意思是“坏的；糟糕的”，bad apple字面意思坏苹果，引申为坏家伙；散布坏影响、起腐蚀作用的人。

【例句】
Before you accuse the entire department of wrongdoing, you should try to find the bad apple that initially caused the problem.
在你指控整个部门做错事之前，先揪出最先导致问题出现的坏家伙吧。

All of the students are well behaved apart from one bad apple who likes to stir up trouble in the class.
全班学生都表现得很好，除了一个害群之马总爱在班里惹是生非。