A "thirty something" that wears clothing resembling that of a teenager. A mother who dresses like her daughter. A man in his midlife dressed as a skater.
There are many Gruppy examples out on the street, just look around you. Usually they are people who are in denial that they are getting older. Clutching desparately to their youth, using appearance as a tool to try and fool those around. This term was recently coined by a New York Fashion Magazine.