Green’s Dictionary of Slang

1. to unlock.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 227: To unbetty, or betty a lock, is to open or relock it, by means of the betty, so as to avoid subsequent detection.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859, 1860		Hotten Dict. of Modern Sl. etc.
1864, 1867, 1870		Hotten Sl. Dict.
1871–82		B.M. Carew Life and Adventures n.p.: unbotty [sic] to unlock.

2. (US Und.) of a prostitute, to unbutton the customer’s trousers.

1939

D. Maurer ‘Prostitutes and Criminal Argots’ in Lang. Und. (1981) 117/2: To unbetty. To unbutton a customer.