A "jupon" in medieval times was a tight-fitting, often padded and quilted garment worn under armor, essentially a late medieval version of the surcoat, primarily used by knights and men-at-arms to catch arrows and weapons that might slide off their breastplate; it was usually made of fabric and could be worn alone or over a mail shirt depending on the period and situation.
A "Jack Chain" in medieval terms refers to a chain made of small, flexible metal links, often sewn onto the sleeves of a padded jacket worn by infantry soldiers, providing protection against slashing sword attacks to the arms, particularly during the late medieval period when full plate armor was not readily available to all soldiers; essentially, it was a protective chainmail sleeve extension for the arms.
