The word is variously derived from the tribute paid by English and Scottish border dwellers to Border Reivers in return for immunity from raids and other harassment. This tribute was paid in goods or labour, in Latin reditus nigri "blackmail"; the opposite is blanche firmes or reditus albi "white rent", denoting payment by silver. Alternatively, McKay derives it from two Scottish Gaelic words blathaich, pronounced (the th silent) bl-aich, "to protect" and mal (“tribute, payment”). He notes that the practice was common in the Highlands of Scotland as well as the Borders.
More likely, from black (adj.) + Middle English mal, male (“rent, tribute”), from Old English māl (“speech, contract, agreement,lawsuit, terms, bargaining”), from Old Norse mál (“agreement, speech, lawsuit”); related to Old English mæðel "meeting, council," mæl "speech," Gothic 𐌼𐌰𐌸𐌻 (maþl) "meeting place," from Proto-Germanic *maþlą, from PIE *mod- "to meet, assemble" (see meet (v.)). From the practice of freebooting clan chieftains who ran protection rackets against Scottish farmers. Black from the evil of the practice. Expanded c.1826 to any type of extortion money. Compare silver mail "rent paid in money" (1590s); buttock-mail (Scottish, 1530s) "fine imposed for fornication."
- (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.
- Payment of money exacted by means of intimidation; also, extortion of money from a person by threats of public accusation, exposure, or censure.
- (English law, historical) Black rent, or rent paid in corn, meat, or the lowest coin, as opposed to white rent, which was paid in silver.
- (transitive) To extort money or favors from (a person) by exciting fears of injury other than bodily harm, such as injury to reputation, distress of mind, false accusation, etc.
- He blackmailed a businesswoman by threatening to expose an alleged fraud.
- (archaic) A tribute paid, usually in kind, to reivers or raiders as a form of protection money.
- Payment of money exacted by means of intimidation.
- To extort money from another by means of intimidation.