The Collaborative International Dictionary of English v.0.48:

Overbalance \O`ver*bal"ance\, v. t.
   1. To exceed equality with; to outweigh. --Locke.
      [1913 Webster]

   2. To cause to lose balance or equilibrium.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Overbalance \O"ver*bal`ance\, n.
   Excess of weight or value; something more than an equivalent;
   as, an overbalance of exports. --J. Edwards.
   [1913 Webster]

WordNet (r) 3.0 (2006):

overbalance
    v 1: weigh more heavily; "these considerations outweigh our
         wishes" [syn: preponderate, outweigh, overbalance,
         outbalance]
    2: cause to be off balance; "It is not desirable to overbalance
       the budget"