Mermaidman's Belt is a belt that goes around Mermaid Man's waist. In the episode: Mermaid Man and Barnacle Boy IV, SpongeBob grabs hold of the belt and shrinks the whole town of Bikini Bottom by accident, which started with Squidward. He also shrunk characters like: Sandy, Patrick, Mother SquarePants, Father SquarePants, Nancy, and other miscellaneous characters.
It also has other functions as seen in other episodes where knobs and buttons are everywhere. When the 'M' is turned around, apparently it switches the Jumbo feature to 'Wumbo'. The accusation was made by Patrick Star, although it is not true in any way because it still shrunk him. Mermaidman wears the belt everywhere when with Barnacle Boy.
Mermaidman's belt is a purple color (sometimes yellow in his early appearances) and it displays a capital 'M' on it in a stylish blue. It looks fairly soft and comfy for Mermaidman to wear, and it looks like it has small holes at the ends of the belt. The 'M' can be easily rotated.