blackmail

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Extortion of money or something else of value from a person by the threat of exposing a criminal act or discreditable information.
2. n. Something of value extorted in this manner.
3. n. Tribute formerly paid to freebooters along the Scottish border for protection from pillage.

Century Dictionary and Cyclopedia

1. n. A tribute of money, corn, cattle, or the like, anciently paid, in the north of England and in Scotland, to men who were allied with robbers, to secure protection from pillage. Blackmail was levied in the districts bordering the Highlands of Scotland till the middle of the eighteenth century.
2. n. Hence Extortion in any mode by means of intimidation, as the extortion of money by threats of accusation or exposure, or of unfavorable criticism in the press. It usually implies that the payment is involuntary, and the ground for demanding it unlawful or pretended and fraudulent.
3. n. Rent paid in produce, or in baser money, in opposition to rent paid in silver.
4. To extort money or goods from, by means of intimidation or threats of injury of any kind, as exposure of actual or supposed wrong-doing, etc. See the noun.

Wiktionary

1. n. archaic 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.
2. n. Payment of money exacted by means of intimidation; also, extortion of money from a person by threats of public accusation, exposure, or censure.
3. n. English law Black rent, or rent paid in corn, flesh, or the lowest coin, a opposed to white rent, which paid in silver.
4. v. transitive To extort money from by exciting fears of injury other than bodily harm, such as injury to reputation, distress of mind, false accusation, etc.; as, to blackmail a merchant by threatening to expose an alleged fraud.

GNU Webster's 1913

1. n. 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.
2. n. Payment of money exacted by means of intimidation; also, extortion of money from a person by threats of public accusation, exposure, or censure.
3. n. (Eng. Law) Black rent, or rent paid in corn, flesh, or the lowest coin, a opposed to “white rent”, which paid in silver.
4. v. United States To extort money from by exciting fears of injury other than bodily harm, as injury to reputation, distress of mind, etc..

WordNet 3.0

1. n. extortion of money by threats to divulge discrediting information
2. v. exert pressure on someone through threats
3. v. obtain through threats