The Collaborative International Dictionary of English v.0.48:

Blackmail \Black"mail`\, n. [Black + mail a piece of money.]
   1. A certain rate of money, corn, cattle, or other thing,
      anciently paid, in the north of England and south of
      Scotland, to certain men who were allied to robbers, or
      moss troopers, to be by them protected from pillage. --Sir
      W. Scott.
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   2. Payment of money exacted by means of intimidation; also,
      extortion of money from a person by threats of public
      accusation, exposure, or censure.
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   3. (Eng. Law) Black rent, or rent paid in corn, flesh, or the
      lowest coin, a opposed to "white rent", which paid in
      silver.
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   To levy blackmail, to extort money by threats, as of injury
      to one's reputation.
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