unpaid prostitute
A more polite way of saying "slut"; remember that a whore gets paid while a slut does the same work for free.
From SNL parody of Girls:
Jessa: I had sex with a cab driver, alright? It's called being free.
Blerta: You are prostitute?
Jessa: No, it's not prostitution if you don't get paid.
Blerta: You are UNPAID prostitute?!? You are lower than dog!
Jessa: I had sex with a cab driver, alright? It's called being free.
Blerta: You are prostitute?
Jessa: No, it's not prostitution if you don't get paid.
Blerta: You are UNPAID prostitute?!? You are lower than dog!