An Aint shit ass nigga is a dude who will never amount to shit. He will never want to amount to shit, never will accidently amount to shit, and is never going to aspire to amount to shit.
Man that dude Chris Hendrix is an ain't shit ass nigga, he ain't got a job or a hustle and his dick is so small it looks like a pussy.
Something you say when you've been dealt a vast amount of ownage. It's a compulsory response to having your ass handed to you. It's usually uttered in the hopes of saving face.