Lester Piggott's Cellmate

Alright, listen up, nimrod! You see, the joke is playin' on words. It's "Lester Piggotts cell mate." Get it? Lester Piggott was a famous jockey and rode a Derby winner. The punchline implies his cellmate also rode (or was ridden by) him. Being as you are probably a mouth breather, I bet you didn't even know Lester Piggott went to jail for tax fraud in 1987.