1550s, "tribute paid to men allied with criminals as protection against pillage, etc.," from black (adj.) + Middle English male "rent, tribute," from Old English mal "lawsuit, terms, bargaining, agreement," from Old Norse mal "speech, agreement;" related to Old English mæðel "meeting, council," mæl "speech," Gothic maþl "meeting place," from Proto-Germanic *mathla-, from PIE *mod- "to meet, assemble" (see meet (v.)).
The word comes from the freebooting clan chieftains who ran protection rackets against farmers in Scotland and northern England. The custom persisted until mid-18c. Black from the evil of the practice. The sense expanded by 1826 to mean any extortion by means of intimidation, especially by threat of exposure or scandal. Compare silver mail "rent paid in money" (1590s); buttock-mail (Scottish, 1530s) "fine imposed for fornication."
"to extort money or goods from by intimidation or threat," especially of exposure of some wrong-doing, 1852, from blackmail (n.). Related: Blackmailed; blackmailing.