I asked that question before too, someone explained to me it's a prison thing (but evolved into something people can just do to modify themselves) called pearling, where they insert beads into the flesh. Each bead symbolizes each year spent in prison. But now they can just do it to their pp so they can stimulate the hole of the person they're fcking.
(Though my personal theory, is that Jingi put it on his pp to purposefully hurt the other prisoners he fcks. And low-key so he can defend himself, because I have some suspicions that Jingi has suffered something traumatic, and that's why he does violent sex with the fat prisoners (٥↼_↼) but that's just my theory which I'm afraid to expand on.)
That theory makes a lot of sense and thanks for explaining cuz I was so confused
I still can’t get over his dick WHY R THERE BALLS