A term used back in the day when the white people saw black people with their pants hanging. So they said his pants are saggin ! And what is "saggin" backwards? "Niggas"! Those tricky white people !
I've heard the jail term..but instead of having a gang behind them, Iheard that it was because in jail, the inmates aren't given belts, and they pant size isn't always considered when they're given their uniforms...it's moved outside of jails, into hip hop, but they're emulating inmates, and it spells "niggas" backwards, rather derogatory and nothing the kids should aspire to be.
saggin originated in prison when prisoners were gay and they would sag to let other inmates know that they took it up the ass. Now only double bags and fuckboy wanna be's sag.