What exactly is a "poly-girl?"?

Answer 1

Depending in what context you heard this phrase, it could mean she is Polynesian or it could mean she is polyamourous.
Polyamoury is a word used by people who do not practice monogomy in their LTR's. They usually have more than one partner in life at the same time. For more information than that you will need to speak to your parents or do some research on the web on your own.