Old Man Prisoner is a character seen in "Gut Grinder." He is being held prisoner by the Spiky People when Jake is thrown in jail. It is unknown why the Old Man Prisoner is in jail, but he claims that it is for the "free grub." He then grabs a grub from the floor, sticks it in his nearly toothless mouth and starts laughing. The grub looks annoyed rather than terrified, so the Old Man Prisoner may have meant grub literally with reference to the one in the cell. Finn takes the Old Man Prisoner's only tooth, a false gold one, to lure the Gut Grinder.
He later pets Jake while repeatedly calling him the Gut Grinder, putting Jake in a trance-like state that makes Jake believe he is the real Gut Grinder.
He looks like a very old man, with grayish skin and a hunched back. He has many wrinkles on his body, supposedly because of his oldness. He also wears a grey, tattered shirt, and equally battered-looking black shorts.
I'm in jail and I'm not the Gut Grinder. I'm here for the free grub.
Aww, Gut Grinder... Aww, Gut Grinder. You're a lil' cutesie, a lil' cutesie wutesie.