Green’s Dictionary of Slang

(US black) second-rate, unfashionable, typified by the use of a backpack, which is far from chic .

2000		Kool Keith ‘Operation Extortion’ 🎵 Why you reviewin it, why you ratin it? You fuckin hatin it, backpack-ass nigga.
2007		W.J. Cobb To the Break of Dawn 155: Common's future rep would be as a ‘backpack’ rapper, a semi-underground artist who catered more to the tofu and patchouli crowd that than to the brew-swilling brothers.