Taking a dump - comes from one of the greatest movies of all time - The Human Centipede. After the three people are attached ass to mouth in a line - the first part of the hum an"centipede" takes a shit. Of course, it goes into the mouth of the second part of the "centipede" while the mad doctor screams "Yah! Feed the bitch".
To your friends while getting up to go take a shit - "Alright guys, I gotta go feed the middle.