Jailmate
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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.
Jailmate meaning & definition 1 of Jailmate.