A small instrument used by house-breakers to force open doors. Bring bess and glym; bring the instrument to force the door, and the dark lantern. Small flasks, like those for Florence wine, are also called betties.
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.
Bermudas * Bess-1Nathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes