The Collaborative International Dictionary of English v.0.48:

Arches \Arch"es\,
   pl. of Arch, n.
   [1913 Webster]

   Court of arches, or Arches Court (Eng. Law), the court of
      appeal of the Archbishop of Canterbury, whereof the judge,
      who sits as deputy to the archbishop, is called the Dean
      of the Arches, because he anciently held his court in the
      church of St. Mary-le-Bow (de arcubus). It is now held in
      Westminster. --Mozley & W.
      [1913 Webster]

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

ARCHES COURT. The name of one of the English ecclesiastical courts. Vide
Court of Arches.