Definition of "arris"

A sharp edge or ridge formed by the intersection of two surfaces  examples

(architecture) A sharp edge or ridge formed by the intersection of two curved surfaces  examples

(archaeology) A ridge formed on the surface of flaked stone, such as a arrowhead or hand axe, as the result of the intersection of two or more flake removals.

(Cockney rhyming slang) Buttocks, arse.