Truffle Legs are an Expert Mode-exclusive accessory from the Treasure Bag dropped from the Truffle Toad. It is a stronger version of the Frog Leg. When equipped it allows increased jump speed, increased jump height,the ability to auto-jump when needed, and immunity to fall damage.
This accessory used to be combined with other 2 accessories dropped from Feudal Fungus and Mushroom Monarch to make a really useful accessory called the Madness Truffle. But it is removed due to granting infinite flight. However, it will return in 4.0 as Mushroom Madness' Expert Mode item.