"Sittin' On The Toilet!" is a popular internet phrase from a video by the same name, published on YouTube by alonzochadwickw.
It is a video of his sister, Nonnie. The story goes that Nonnie posted an embarrassing photo of Alonzo on her MyspaCEO two years prior. He swore he would get her back. So, when Nonnie used Alonzo's camera to make this random video, as she was bored, and forgot to delete it, Alonzo posted it on his YouTube channel, calling it 'THE ULTIMATE REVENGE'. 
Later, it was re-used in many, many Try Not To Laugh Videos, Vine Compilations, Remixes, and more.