From the Super Mario Wiki
A Goomnut is a chestnut that appears in Paper Mario. Mario can only get them by hitting the Goomnut Tree near Goombario's house in Goomba Village with his Hammer. When Mario fights the Goomba King, there will be a Goomnut Tree in the background. Mario can hit the tree's bandaged root to make a huge Spikey Goomnut (resembling a chestnut still inside the spiny shell) hit the King and the Red and Blue Goombas. (Similarly, though, the Goomba King himself can also attack this root, sending a barrage of smaller Spikey Goomnuts to attack Mario and Goombario.)
A Goomnut heals three FP and Tayce T. can cook one to make a Nutty Cake. It is used mostly for recipes and is one of the few receivable items at the beginning of the game, resulting in its main use as an item coming then. Mixing Goomnuts with Super Shrooms is an easy way to get Life Shrooms.
In Luigi's Diary, Luigi mentions the Goomba family in Goomba Village giving him a Goomnut. He states that he recovered 3 FP after using it and then asks his diary, "Do you think Mario will notice?"
Names in other languages
|"Big Round Chestnut". It is also a pun on クリボー Kuribō, meaning "Goomba", and 栗 kuri, another word for "Chestnut".