c. 1300, "absolute ruler," especially one without legal right; "cruel, oppressive ruler," from Old French tiran, tyrant (12c.), from Latin tyrannus "lord, master, monarch, despot," especially "arbitrary ruler, cruel governor, autocrat" (source also of Spanish tirano, Italian tiranno), from Greek tyrannos "lord, master, sovereign, absolute ruler unlimited by law or constitution," a loan-word from a language of Asia Minor (probably Lydian); Klein compares 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"]
Originally in Greek the word was not applied to old hereditary sovereignties (basileiai) and despotic kings, but it was used of usurpers, even when popular, moderate, and just (such as Cypselus of Corinth), however it soon became a word of reproach in the usual modern sense. The unetymological spelling with -t arose in Old French by analogy with present-participle endings in -ant. Fem. form tyranness is recorded from 1590 (Spenser); Medieval Latin had tyrannissa (late 14c.).