When a guy sees a girl who he thinks is attractive, he may show signs of this by staring or saying something. If someone else thinks the girl is underage, they may exclaim "Jailmate." It refers to the fact that if the guy who originally spotted the girl actually made a move for her, he would go to jail; as she is underage.
John: "Christ, she's fit!"
Chris: "Jailmate, I'm not sure she's done her GCSE's."
Alex: "Look at that Steve!"
Steve: "Jailmate, she's fuckin' 15"
Chris: "Jailmate, I'm not sure she's done her GCSE's."
Alex: "Look at that Steve!"
Steve: "Jailmate, she's fuckin' 15"
by Snow Nicholas January 10, 2019