A person who threatens to reveal embarrassing or damaging secrets unless someone gives them money or does what they want.
From 'black' (meaning evil or dark) and 'mail' (from Scots/Middle English, meaning 'payment' or 'rent'). Scottish bandits would demand 'blackmail' (protection money), and the term evolved to describe extortion through threats.
Blackmail started as actual protection money paid to Scottish highland raiders, which is why it stuck as the name for extortion—criminals literally named their own crime, and the name was so good it's still used 400 years later.
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