“ 爛蘋果 ”

If you have one spoiled （爛的） apple in a box with a lot of good apples, the bad one will soon damage the others. Because a bad person can have a negative influence on those around him or her, he or she is called a bad / rotten apple.
　　如果你把一個爛蘋果和一堆好蘋果都放在盒子裡，那麽這個壞蘋果會很快也讓其他的受到傷害，正如一個壞人會帶壞他身邊的一群人一樣，那麽他/她就被稱爲害群之馬。

　　"Every school has a couple of bad apples who damage its　reputation （聲譽）," Danny said."Joseph is the rotten apple in our school."
　　“每個學校都有一些害群之馬在損害學校的聲譽，”丹尼說，“約瑟夫就是我們學校的害群之馬。”