'Joint Body' was common slang used by members of law enforcement, especially those working undercover to identify and snuff out ex-cons and former felons. The term refers to the physique that an ex-con yields while being in the "joint." This coupled with a certain strut, or walk on the outside making themselves appear strong and confident. The "body" portion refers to the lean muscle mass an ex-con accumulates while in the "joint," via lifting weights.
"I can spot a joint body coming from a mile away."