mid-14c., pokete, "small bag or pouch, small sack," from Anglo-French pokete (13c.), diminutive of Old North French poque "bag" (Old French pouche), from a Germanic source akin to Frankish *pokka "bag," from Proto-Germanic *puk- (see poke (n.1)).
The narrower meaning "small bag worn on the person, especially one sewn into a garment" is from early 15c. The sense of "one of the small bags or nets at the corners and sides of some billiards tables" is from 1754. The mining sense of "cavity in the ground filled with ore" is attested from 1850; the military sense of "area held by troops almost surrounded by the enemy" is from 1918; the general sense of "small area different than its surroundings" (1926) apparently was extended from the military use.
Figuratively, "one's money" (conceived as being kept in a pocket), from 1717; hence to be out of pocket "expend or lose money" (1690s); Pope Pokett (late 15c.) was figurative of the greedy and corrupt Church.