The Poo Radley desends from Boo Radley from To Kill A Mockingbird. Boo was changed to Poo cause it sounds smellier and it is more insulting.
The whole term is an insult coming from the sentence: "You smell of Poo Radley's Clitoris" or " You smell of PR's C" for short.
Person B: "Well you smell of Poo Radley's Vagina"
Person A: "Well you smell of Poo Radley's Clitoris" "Yay I win and you smell of PR'sC"