Derived from the early teachings of the church:
A person who has had sex before marriage.
Over the years, it has evolved into what it is today -- a female with loose morals or simply a woman who enjoys sex without being tied to one partner.
Also the term 'slut' is used by guys who can't get laid or get refused sex by a particular girl. Or something used to destroy a girls reputation/credibility (some girls cut down or refuse sex due to the fear of being labelled a 'slut').
I didn't want to fuck her anyway cuz she's a slut.
Don't pay attention to her cuz she's a slut.
Some Russian girls that went to Turkey to make money ended up prostituting and branded themselves as 'Natasha'. Now some Turks refer to sluts as Natasha's.