The Collaborative International Dictionary of English v.0.48:

Approver \Ap*prov"er\, n.
   1. One who approves. Formerly, one who made proof or trial.
      [1913 Webster]

   2. An informer; an accuser. [Obs.] --Chaucer.
      [1913 Webster]

   3. (Eng. Law) One who confesses a crime and accuses another.
      See 1st Approvement, 2.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Approver \Ap*prov"er\, n. [See 2d Approve, v. t.] (Eng. Law)
   A bailiff or steward; an agent. [Obs.] --Jacobs.
   [1913 Webster]

WordNet (r) 3.0 (2006):

approver
    n 1: an authority with power to approve

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

APPROVER, Engl. crim. law. One confessing himself guilty of felony, and
approving others of the same crime to save himself. Crompt. Inst. 250 3
Inst. 129.