arris

Etymology 1

From Old French areste, from Latin arista (“beard (of grain), fishbone”). Doublet of arista.

noun

1. A sharp edge or ridge formed by the intersection of two surfaces
2. (architecture) A sharp edge or ridge formed by the intersection of two curved surfaces
3. (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.

Etymology 2

Rhyming slang via Aristotle = bottle, then bottle and glass, then arse.

noun

1. (Cockney rhyming slang) Buttocks, arse.