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]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "tyrannize":
   beat down, bend, break, break down, bring low, bring to terms,
   browbeat, bulldoze, bully, castrate, clamp down on, coerce, compel,
   conquer, cow, crush, daunt, despotize, dictate, dominate, domineer,
   domineer over, enslave, grind, grind down, henpeck, humble,
   humiliate, intimidate, keep down, keep under, lord it over, master,
   oppress, order about, overawe, overbear, overlord, overmaster,
   override, overwhelm, press heavy on, quell, reduce, repress,
   ride over, ride roughshod over, shackle, subdue, subjugate,
   suppress, terrorize, trample, trample down, trample underfoot,
   trample upon, tread down, tread underfoot, tread upon,
   tyrannize over, unman, walk all over, walk over, weigh heavy on