A seemingly useless item. It's listed as armor, but as the description says, there's no sleeve slot on your character. Yet you can use it in some recipes, like of the Magical Badass Jacket.
However using right click as you would to equip an item removes the sleeve from ones inventory permanently. Stats found on the sleeve do not transfer to the character when this is done, nor do they transfer to the Magical Badass Jacket if it is used in crafting.
Don't confuse it with the Wizards' Sleeve.