BETTY, BESS,
a small Engine to force open the Doors of Houses; Mill the Gig with your Betty; i.e. Break open the Door with your Instrument.
· 
· 
Entry taken from 1736 Canting Dictionary, by Nathan Bailey.
The Vulgar Tongue originally collected by Grose, 1811.
Also see examples in Canting Songs and Slang Rhymes collected by John S. Farmer (1896).