When a person has a buff, muscular upper body, but skinny chicken legs.
The term originated from the appearance of prison inmates who spend hours lifting weights to build up their chest, back, and arms, but neglect building up their leg muscles entirely.
"Hey man, you can't just only do bench presses and bicep curls in the gym all the time or you're gonna end up looking prison yoked."
"My cousin just got out on parole last night. He looks all prison yoked now.