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).
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.).