Poly is a person who is in love with multiple people or in a relationship with multiple people so basically just a cheater and a fuckgirl or a fuckboy who can’t be loyal.
Using "poly" is ill-advised. It could mean polyamorous, but it could be the very racist shorthand for Polynesian. If you say it around anyone from the Pacific, you might get punched.
"Those steam punks are all poly."
"What did you just say?"
"I said they're all poly. They like to sleep around."