An old-school saying referring to the fact that a couple is living in sin until they're married. Once married, the bride would be considered an honest woman.
"Joe, you've been dating her for seven years. When are you going to get married and make an honest woman out of her?"
A female human that can actually be trusted to tell you the truth! This usually results from a freak head-injury or an extremely rare genetic anomaly. In the highly unlikely event you ever find one, do your best to keep her because she is rarer than the four-leafed clover and even more valuable than a four-bedroom house in London!
The last girl I slept with told me she was an honest woman because she was thinking of dumping her boyfriend anyway.