The Dragon Chestplate is an item that is part of the complete Dragon Armor, once worn by the First Spinjitzu Master, before he left the First Realm. It was located in Firstbourne's Nest, until it was found by the Iron Baron and Wu, and the latter now wears it.
When Iron Baron and Wu made it to the nest, he stole the armor and tried to control the Firstbourne. However, Wu told him it was not the armor that controls her, and she promptly buries him in molten rock. Wu then took possession of the Armor.
Wu wore the Dragon Armor during his fight with Garmadon.