Specifically, a monarch, or other ruler or master, who uses
power to oppress his subjects; a person who exercises unlawful
authority, or lawful authority in an unlawful manner; one who by
taxation, injustice, or cruel punishment, or the demand of unreasonable
services, imposes burdens and hardships on those under his control,
which law and humanity do not authorize, or which the purposes of
government do not require; a cruel master; an oppressor.

Any one of numerous species of American clamatorial birds
belonging to the family Tyrannidae; – called also tyrant bird.

To act like a tyrant; to play the tyrant; to tyrannical.