Definition of "brassart"

Itobad, always surprised at the disgraces that happened to such a man as he, was obliged to yield to Zadig, who took from him with great composure his magnificent helmet, his superb cuirass, his fine brassarts, his shining cuishes […]

1762, T. Smollett, T. Franklin and others (translators), "Zadig", in The Works of M. de Voltaire, page 222

[…] cross-bow glanced away from the well rivetted plates, and the stroke of the sword rang harmlessly upon the helmet, the brassarts and the cuisses of proof. While infantry were powerless and destitute of physical solidity […]

1779, Miscellanea Critica: Essays & Comment on Current Topics, page 228