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