Where prisoners who were willing to have sex with other prisoners needed to invent a signal that would go unnoticed by the guards so they wouldn't suffer consequences. So, by partially showing their butts, they showed that they were available to be penetrated by other inmates.

Don't believe it, Check out the Urban Dictionary Entry for Low Riders

"Refers to homosexual men wearing their pants low on the hip to expose colorful underwear in an attempt to attract a mate. The low riding pants signifies easy access to the mans rear compartment and not so subtly letting the other man know that he is on the down low."

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