Your girlfriend's friends that constantly talk shit about you and desire nothing more than to a.) Get your girlfriend to break up with you or b.) Fuck around behind your back.
Most often, the Girlfrienemy is grossly overweight and chronically single. Dragging your girlfriend into her misery is her only source of joy.
John knew he had a new Girlfrienemy when his girlfriend started hanging out with the fat chick from work and arguments inevitably took place the day after about how shitty a boyfriend he had become.
Super hot girlfriend that's out to stab your back but for some reason keeps on kissing you and telling you she will never leave.
Names chosen randomly
Mark: My girlfriend just cheated on me.
Dave: That sucks. With who?
Mark: With my cousin.
Dave: Why? She hates your cousin!
Mark: She was pushing for that new job downtown in the corner office for me.
Dave: What's the problem then?
Mark: She's interested in the man my boss will be.
Dave: She's a real Girlfrienemy isn't she Mark?