I have heard 'doxy' used as a slang word for 'sweetheart', years ago in college, by students with a perverted sense of humor, but doubt its currency anywhere now.
I have an 1850s reference from an American minister who used the word in the sense of harlot.
The American "Webster's Collegiate Dictionary" only has the meaning of floozie or prostitute.

The word has become largely obsolete except in literary-historical usage.