a scrub is a guy that thinks he's fly, also known as a busta. Always thinkin' 'bout what he wants and just sits on his broke ass. Sittin' in the passengers side of his best friend's ride, trying to holla. If you don't have a car and you're walking, you live at home with your mama, and/or have a shawty and don't show love, you're a scrub/busta.
The best example can be heard in the TLC song "No Scrubs."