A Proselyte cuisse is the platelegs part of the Proselyte armour set that can be bought from Sir Tiffy Cashien in Falador after completing the Slug Menace quest. The quest is a requirement for wearing the cuisse, along with 30 Defence and 20 Prayer.
It has the exact equivalent defence bonuses as Mithril platelegs, but also offers +5 Prayer bonus. This prayer bonus is equal to the Monk's robe (bottom). It is white with dark gold trim.
This piece of armour cannot be made using the Smithing skill.
The cuisse, if part of a full set of the armour, can be stored in the Armour case of a Player-owned house.