From the Super Mario Wiki
A Deku Nut is an item from the The Legend of Zelda series, which also appears once again as an item in Super Smash Bros. Brawl. When thrown at an opponent, it will stun the opponent for a few seconds. The blast radius can affect several fighters at once. The effect is similar to players breaking other players' shields (or their own). It is possible for the thrower of the item to also be stunned during the explosion of the Deku Nut. If players are in mid-air during the explosion, they will be blasted away, similar to Bob-ombs. Deku Nuts will also explode on their own if left alone for a while. If picked up and held, they will still explode after a short while, stunning the holder of the item. In the The Legend of Zelda series, starting with The Legend of Zelda: Ocarina of Time, they could be used to stun an enemy.
Super Smash Bros. Brawl Trophy Information
| Deku Nuts
|| N64 The Legend of Zelda: Ocarina of Time
|| Items that explode in a flash of blinding light shortly after they appear or when they take damage. Those caught in a blast will be dazed and immobile for a few moments. If the item is thrown, it has the same effect on whomever it hits. In The Legend of Zelda: Ocarina of Time, Deku Nuts stunned enemies and were indispensable when Link was surrounded.