Why is the ninth month called September?
c.1300, "absolute ruler," from Old French tyrant (12c.), from Latin tyrannus "lord, master, tyrant" (cf. Spanish tirano, Italian tiranno), from Greek tyrannos "lord, master, sovereign, absolute ruler," a loan-word from a language of Asia Minor (probably Lydian); cf. Etruscan Turan "mistress, lady" (surname of Venus).
In the exact sense, a tyrant is an individual who arrogates to himself the royal authority without having a right to it. This is how the Greeks understood the word 'tyrant': they applied it indifferently to good and bad princes whose authority was not legitimate. [Rousseau, "The Social Contract"]The spelling with -t arose in Old French by analogy with present participle endings in -ant. Fem. form tyranness is recorded from 1590 (Spenser); cf. Medieval Latin tyrannissa (late 14c.).