The Collaborative International Dictionary of English v.0.48:

Overmaster \O`ver*mas"ter\, v. t.
   To overpower; to subdue; to vanquish; to govern.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overmaster
    v 1: overcome by superior force [syn: overpower, overmaster,
         overwhelm]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "overmaster":
   beat down, bend, break, break down, bring low, bring to terms,
   browbeat, bulldoze, bully, castrate, clamp down on, coerce, compel,
   conquer, cow, crush, daunt, deck, despotize, domineer,
   domineer over, enslave, floor, grind, grind down, henpeck, humble,
   humiliate, intimidate, keep down, keep under, lord it over, master,
   oppress, overawe, overbear, overcome, overmatch, overpower,
   override, overset, overthrow, overturn, overwhelm, press heavy on,
   quell, reduce, repress, ride over, ride roughshod over,
   send flying, silence, subdue, subjugate, suppress, surmount,
   terrorize, trample down, trample underfoot, trample upon,
   tread down, tread underfoot, tread upon, trip, trip up, tyrannize,
   tyrannize over, unman, upset, walk all over, walk over,
   weigh heavy on