Meaning

noun extortion of money by threats to divulge discrediting information

verb exert pressure on someone through threats

verb obtain through threats

originFrom black + mail (“a piece of money”). Compare Middle English blak rente (“a type of blackmail levied by Irish chieftains”). 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 (“black mail”); the opposite is blanche…

Etymology from Wiktionary, CC BY-SA 3.0