ShakespearesWords.com

jack (n.)

Old form(s): Iacks

jacket, tunic, coat [usually of quilted leather]

1H4 IV.ii.47	[Prince Hal to Falstaff] How now, blown Jack? How now, quilt?
E3 I.ii.29	[King David to Lorraine, of the Scots soldiers] never shall [they] ... lay aside their jacks of gimmaled mail