Arris

[Sarah]

The sharp edge or salient angle formed by two surfaces meetingeach other, whether plane or curved; -- applied particularly to theedges in moldings, and to the raised edges which separate theflutings in a Doric column. P. Cyc. Arris fillet, a triangular pieceof 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 abuilding. Gwilt.

[Sarah]

arse. from cockney rhyming slang -
1 - arse becomes 'bottle and glass' which becomes bottle (proper rhyming slang normally leaves out the rhyming word, being designed to [obfuscate] meaning)
2 - bottle becomes aristotle
3 - which gets shortened to arris

[Greta]

A sharp edge formed by the meeting of two flat or curved surfaces.