The Collaborative International Dictionary of English v.0.48:

Misericordia \Mis`e*ri*cor"di*a\, n. [L., mercy, compassion;
   miser wretched + cor, cordis, heart.]
   [1913 Webster]
   1. (O. Law) An amercement. --Burrill.
      [1913 Webster]

   2. (Anc. Armor.) A thin-bladed dagger; so called, in the
      Middle Ages, because used to give the death wound or
      "mercy" stroke to a fallen adversary.
      [1913 Webster]

   3. (Eccl.) An indulgence as to food or dress granted to a
      member of a religious order. --Shipley.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

MISERICORDIA, mercy. An arbitrary or discretionary amercement.
     2. To be in mercy, is to be liable to such punishment as the judge may
in his discretion inflict. According to Spelman, misericordia is so called,
because the party is in mercy, and to distinguish this fine from
redemptions, or heavy fines. Spelm. GI. ad voc.; see Co. Litt. 126 b, and
Madox's Excheq. c. 14. See Judgment of Misericordia.