The chief or principal person involved in planning a secret plot or conspiracy against someone.
From arch- (chief, principal) + conspirator (from Latin conspirare, 'to breathe together' or 'plot together'). The arch- prefix indicates the leader or most important member of the conspiracy.
Shakespeare loved using 'archconspirator' to describe the head villain in his plays—it's the perfect word for the mastermind you discover at the end who was pulling all the strings the whole time!
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