Someone who, while on AIM, constantly copies parts of their IMs with certain people and pastes them to others. Like in real life, the big mouth will rat people out when they talk about someone else, only this time it's online rather than through spoken words.
My brother is the biggest AIM big mouth in the world. Every time I talk about someone, he has to copy and paste that excerpt of our conversation and send it to them. One day, he will pay for this.