A hussy is a woman or girl who's disrespectful or immoral. Your old fashioned grandmother might call your brother's live-in girlfriend a hussy.
The noun hussy is an insulting and somewhat antique way to refer to a woman whose actions don't line up with traditional ideas of being respectable. A sexually bold women is most likely to be called a hussy, especially if she's seen as "stealing" another woman's partner. Originally, hussy was a shortening of the Middle English husewif, "housewife." Through the 1500's, hussy came to mean "any woman or girl," and by the 1650's it meant "an improper woman or girl."