Jupon

a short, fitted surcoat, worn over armour in the 14th and early 15th centuries. Made of several thicknesses of fabric, the other layer was often a rich velvet or silk, with the owner’s arms embroidered or appliqued on.

see Gipon

♦ Tightly fitted garment worn over armour in the fourteenth century.

(Prestwich, Michael. Armies and Warfare in the Middle Ages: The English Experience, 348)

♦ Short leather tunic worn over chain mail.

(Seward, Desmond. Henry V: The Scourge of God, 223)

♦ Sleeveless, hip-length garment of leather or padded textile worn over a knight's armour and blazoned with his coat of arms.

(Wise, Terence. Medieval Warfare, 249)