To begin with, the definition of the word 'pocket' states that it is 'a small baglike attachment'. The reason for this particular definition is that the pocket was not originally sewn into garments as it is today. In fact, the first pockets were actually small pouches that hung from the belt where one could carry valuables and coins. The word itself comes from the Anglo-Norman word pokete and traces its roots to the Germanic root word 'bag', which is like the Old English word pocca . Therefore, the definition makes sense. 'Purse' and 'pocket', incidentally, have the same root word, only one is plural and the other singular.
In addition and worthy of note is the Scottish sporran , which is that nifty purse worn at the front of the kilt in traditional wear. The word sporran itself comes from the old Irish word spar