Arches \Arch"es\, pl. of Arch, n.

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.

Source: Webster's Revised Unabridged Dictionary (1913)