Noun

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.
2. Payment of money exacted by means of intimidation; also, extortion of money from a person by threats of public accusation, exposure, or censure.
3. (English Law): Black rent, or rent paid in corn, flesh, or the lowest coin, a opposed to white rent", which paid in silver. To levy blackmail, to extort money by threats, as of injury to one's reputation.

Verb

1. : To extort money from by exciting fears of injury other than bodily harm, as injury to reputation, distress of mind, etc.; as, to blackmail a merchant by threatening to expose an alleged fraud.