mid-14c., pokete, "bag, 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)).
Meaning "small bag worn on the person, especially one sewn into a garment" is from early 15c. Sense in billiards is from 1754. Mining sense is attested from 1850; military sense of "area held by troops 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) is from 1717. Pope Pokett (late 15c.) was figurative of the greedy and corrupt Church.