Pygist, noun. (Greek pygo- rump, buttocks + -ist)
One who engages in anal sex.
"Pygist, one that useth buggerie." - Henry Cockeram, English Dictionarie: or, an interpreter of hard English words, 1623
Apparently I am not far enough removed from jr. high school that I can refrain from laughing that the first known usage of this word was by a man named "Cockeram."