Green’s Dictionary of Slang

1. (UK Und.) a short iron bar, used to break open doors, force locks etc.

c.1698		B.E. Dict. Canting Crew n.p.: Bess, c. bring bess and glym, c. forget not the Instrument to break open the door and the Dark-lanthorn.
1719		A. Smith Lives of Most Notorious Highway-men, etc. (1926) 202: [as cit. c.1698].
1725		New Canting Dict. n.p.: bess c. Bring Bess and Glym; i.e. Forget not the Instrument to break open the Door and the Dark-lanthorn [...] betty or 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.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1768		(con. 1710–25) Tyburn Chronicle II in Groom (1999) xxix: A Bess or Betty A small Iron Crow.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1862		E. de la Bédollière Londres et les Anglais 313/1: bess, [...] instrument pour forcer les portes.

2. a picklock.

1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 10: Betty, or Bess — a crooked nail to open locks. Several sizes are carried by cracksmen: they are bent first and hardened afterwards.
1859		Matsell Vocabulum 10: bess A pick of a very simple construction.
1881		Trumble Sl. Dict. (1890).
1895		Crowe Aus. Sl. Dict. 8: Bess [...] pick-locks.