black·mail 
(blăkmāl′)
(blăkmāl′)n.
1.
a. Extortion of money or something else of value from a person by the threat of exposing a criminal act or discreditable information.
b. Something of value, especially money, extorted in this manner: refused to pay blackmail.
2. Tribute formerly paid to freebooters along the Scottish border for protection from pillage.
[BLACK + MAIL3.]
blackmail′ v.
blackmail′er n.