This is a mug shaped like the Necronomicon, a book first introduced in the Cthulhu Mythos by H.P. Lovecraft, and portrayed here as envisioned by Big Toe.

The Necronomicon, also referred to as the Book of the Dead, or under a purported original Arabic title of Kitab al-Azif, is a fictional grimoire (textbook of magic) that is sentient and evil -- said to be bound in human skin.