A name of contempt given to a man who interferes with the duties of female servants, or occupies himself in female pursuits; also called a “Molly.”
A skeleton key; the servant of a picklock. Burglars call their short crowbars for forcing locks Jennies and Jemmies. “Jenny” is a “small engine,” i.e. 'ginie, and Jemmy is merely a variant.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Betty from Infoplease: