A pocket. To buy the sack: to get drunk. To dive into the sack; to pick a pocket. To break a bottle in an empty sack; a bubble bet, a sack with a bottle in it not being an empty sack.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Sacheverel * Sad DogNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes