Source : Webster's Revised Unabridged Dictionary (1913)

Amerce \A*merce"\, v. t. [imp. & p. p. {Amerced}; p. pr. & vb.
   n. {Amercing}.] [OF. amercier, fr. a merci at the mercy of,
   liable to a punishment. See {Mercy}.]
   1. To punish by a pecuniary penalty, the amount of which is
      not fixed by law, but left to the discretion of the court;
      as, the amerced the criminal in the sum on the hundred
      dollars.

   Note: The penalty of fine may be expressed without a
         preposition, or it may be introduced by in, with, or
         of.

   2. To punish, in general; to mulct.

            Millions of spirits for his fault amerced Of Heaven.
                                                  --Milton.

            Shall by him be amerced with penance due. --Spenser.

Source : WordNet®

amerce
     v 1: punish with an arbitrary penalty
     2: punish by a fine imposed arbitrarily by the discretion of
        the court