A jack is a padded coat worn as a defensive garment. They might be made of multiple layers of a fabric like linen or fustian (then a combination of wool and linen) and others might be stuffed with tow or rags.
Jack (Drinking Vessel)
A jack was a large hardened leather jug lined with pitch used to store and pour liquids. These long lasting items and were extremely common from both ends of the social spectrum. It is thought that the exclamation that "Englishmen drank from their boots" is derived from the use of this article.