From Brotato Wiki
Hitting an enemy removes +10% of their speed Max: 30%
Ugly Tooth is an in-game item that can be bought from Shop or can be found in a Crate. Most Items increase the Stats of the player while some of them provide unique effects.
Ugly Tooth is a unique item. Unique items can only be bought or found once per run.
Ugly Tooth slows down enemies when you hit them. The slow stacks up to 3 times for a total of a 30% slow.
The slowdown works on Elites and Bosses, and can make a large difference for those fights.
Ugly Tooth can also be rather powerful with Elemental builds that relies on Burning Damage to kill enemies.
Structures such as Turret or Landmines also applies the slow when they hit a target.