The Collaborative International Dictionary of English v.0.48:

Arris \Ar"ris\, n. [OF. areste, F. ar[^e]te, fr. L. arista the
   top or beard of an ear of grain, the bone of a fish.] (Arch.)
   The sharp edge or salient angle formed by two surfaces
   meeting each other, whether plane or curved; -- applied
   particularly to the edges in moldings, and to the raised
   edges which separate the flutings in a Doric column. --P.
   Cyc.
   [1913 Webster]

   Arris fillet, a triangular piece of wood used to raise the
      slates of a roof against a chimney or wall, to throw off
      the rain. --Gwilt.

   Arris gutter, a gutter of a V form fixed to the eaves of a
      building. --Gwilt.
      [1913 Webster]