Blackmail, a certain rate of money, corn, cattle, or the like, anciently paid, in the N. of England and in Scotland, to certain men who were allied to robbers, to be protected by them from pillage. It was carried to such an extent as to become the subject of legislation. Blackmail was levied in the districts bordering the Highlands of Scotland until the middle of the 18th century. In the United States, the word is applied to money extorted from person under threat of exposure for an alleged offense; hush-money.
Entry from Everybody's Cyclopedia, 1912.