Blackmail

敲诈;勒索
practice blackmail
进行勒索 (来源：英语资料下载 http://download.englishcn.com)

The word "blackmail" became popular in the 1800's and comes from the clan chieftains who ran protection rackets on farmers in Scottland. If the farmers didn't pay the mail (an old term for rent), the chieftains would steal their cattle and crops. Since this was considered evil, it was considered "black".
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(来源：英语电影下载 http://bt.englishcn.com)