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]

The Collaborative International Dictionary of English v.0.48:

Tyrannize \Tyr"an*nize\, v. t.
   To subject to arbitrary, oppressive, or tyrannical treatment;
   to oppress.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tyrannize
    v 1: rule a country as a tyrant [syn: tyrannize, tyrannise,
         grind down]
    2: rule or exercise power over (somebody) in a cruel and
       autocratic manner; "her husband and mother-in-law tyrannize
       her" [syn: tyrannize, tyrannise, domineer]