The Collaborative International Dictionary of English v.0.48:

Tyrannize \Tyr"an*nize\ (t[i^]r"an*n[imac]z), v. i. [imp. & p.
   p. Tyrannized (t[i^]r"an*n[imac]zd); p. pr. & vb. n.
   Tyrannizing (t[i^]r"an*n[imac]`z[i^]ng).] [Cf. F.
   tyranniser, Gr. tyranni`zein to take the part of tyrants.]
   To act the tyrant; to exercise arbitrary power; to rule with
   unjust and oppressive severity; to exercise power over others
   not permitted by law or required by justice, or with a
   severity not necessary to the ends of justice and government;
   as, a prince will often tyrannize over his subjects; masters
   sometimes tyrannize over their servants or apprentices.
   [1913 Webster]