Someone who is poo in a bottle. Generally, they are called this when they irritate someone or tease someone. You're pretty much being called shit...Contained shit...
When you put on latex gloves and open your mate's arsehole and notice the lack of poo inside. Therefore, making you want to fill it up by sharing some of your own poo.
Dr. Riley opened up Fred's anus and noticed that all he saw was red and no poo, so Dr. Riley quickly became Dr. Poolittle and filled up Fred's empty hole.