WHAT IS IT?
Once upon a time courtesan was merely a term for a woman who attends the court of a king or person of similar power. Before the Renaissance they were used to pass along information untrusted to servants to visiting dignitaries.
During the Renaissance the term courtesan referred to the rulers mistress and/or a well educated and independent woman of free morals. Eventually becoming the name of trained singers and dancers..  before coming to be the common euphemism for a very upper class prostitute.
WHERE DO YOU GET IT?
At court, both royal and urban. Wherever there are secrets whispered between the sheets and scheme walks hand in hand with plot.
They forever walk a knifes edge, never knowing if today will be the day that their usefulness and beauty will not be enough to keep the right people from remembering exactly what it is that she knows and opening talks with the wrong people.exactly when they will outlive their beauty or usefulness to be cast aside in favor of another.
USING IT IN REAL LIFE:
The courtesan label brings up visions of high class and elegant women who are able to handle any situation that develops. Whether it be entertaining one particular patron or a group of visiting businessmen. I can highly recommend the book Memoirs of a Geisha by Arthur Golden  for a behind the scenes peek into this life.
USING IT IN WRITING:
- Thanks to the mercurial nature of politics any courtesan is going to be a survivor and adept at thinking on her feet. She will have no hesitation is throwing people to the wolves in order to save her own skin. Life is hard enough and look out for number one is her credo.
- It could be that the person in power has a new favorite and has thrown the once powerful courtesan out into the cold. This leaves her free to plot revenge on those who scorned her 
- It might be that the courtesan is planning to leave the court having found true love.
- Maybe the lovers attempt to legitimize the relationship is making waves with others who were prepared to tolerate her but now that she is making serious inroads into their society and/or lives it is time to do something.
MIGHT ONE CONQUER THE WORLD WITH IT?