mammellière

from The Century Dictionary.

• noun A piece of armor, usually a circular or nearly circular plate, attached to the hauberk or broigne, or worn outside the surcoat, one covering each breast, and serving especially for the attachment of the end of the chain which was secured to the sword-hilt, mace, war-hammer, etc.
• noun The pectoral, especially when serving to retain the ends of the chains securing the sword-hilt, dagger-hilt, or the like, and differing from the piece of armor above defined in being one plate only instead of one of two side by side.