The Mint is an item that can be acquired from the Pawn Shoppe on the Isle of the Crown. It is used to fool Grump-Frump on the Isle of Wonder. Shamir Shamazel is addicted to them, and after eating one begins to act strangely, as if drunk.
On the Isle of the Sacred Mountain, Alexander can acquire a Peppermint leaf from the small cave at the plateau. It is optional, but required for full points. The leaf can be used on Shamir to destroy him, albeit resulting in the bad ending.