The Collaborative International Dictionary of English v.0.48:

Countervail \Coun`ter*vail"\ (koun`t?r-v?l"), v. t. [imp. & p.
   p. Countervailed (-v?ld); p. pr. & vb. n.
   Countervailing.] [OF. contrevaloir; contre (L. contra) +
   valoir to avail, fr. L. valere to be strong, avail. See
   Vallant.]
   To act against with equal force, power, or effect; to thwart
   or overcome by such action; to furnish an equivalent to or
   for; to counterbalance; to compensate.
   [1913 Webster]

         Upon balancing the account, the profit at last will
         hardly countervail the inconveniences that go allong
         with it.                                 --L'Estrange.
   [1913 Webster]