The Collaborative International Dictionary of English v.0.48:

Rebutter \Re*but"ter\ (r[-e]*b[u^]t"t[~e]r), n. (Law)
   The answer of a defendant in matter of fact to a plaintiff's
   surrejoinder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rebutter
    n 1: a debater who refutes or disproves by offering contrary
         evidence or argument [syn: rebutter, disprover,
         refuter, confuter]
    2: (law) a pleading by the defendant in reply to a plaintiff's
       surrejoinder [syn: rebutter, rebuttal]

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

REBUTTER, pleadings. The name of the defendant's answer to the plaintiff's
surrejoinder. It is governed by the same rules as the rejoinder. (q.v.) 6
Com. Dig. 185.