Yours, Etc.: Origins and Uses of 8 Sign-Offs
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.).