Pockets quite often referred to the cloth pouch worn suspended from a belt, under a tunic, or in the case of ladies, under a skirt or petticoat (Lucy Locket lost her pocket). They didn't actually become part of the trouser until the end of the 18th Century.
A pocket knife could possibly be construed as meaning a small, personal knife for food purposes, that could be carried in a pouch or 'pocket'.