A group of people(typically back in the 50's - 70's) who got their names from the "grease" or olive oil they used to slick back their hair with. They go in groups but are not gangs, they will use anything as a weapon and make a living by doing whatever they gotta do.
"Hey look at that greaser over there!"
"Hey grease! why don't you come over here and let me cut that hair!"