Legionnaire's Plate Hauberk

The Legionnaire's Plate Hauberk is the plate chestpiece of the Horde PvP-Rare Warrior set called the Champion's Battlearmor.

Source
This item can be purchased from First Sergeant Hola'mahi located in the Hall of Legends in Orgrimmar. It costs 6750 Honor Points and 30 Arathi Basin Marks of Honor.