Pygo, or πυγο, is the combining form of πυγη, the Greek word for buttocks or rump, and is thus the source of many of the most wonderful words in the English language, including...

callipygian - having shapely buttocks

dasypygal - having hairy buttocks

platypygous - having flat buttocks

pygal - of or pertaining to the buttocks or rump

pygalgia - a pain in the ass

pygarg - a creature with white buttocks

pygist - one who engages in anal sex

pygobranchiate - referring to fish that have gills around the anus

pygopage - conjoined twins conected at the buttocks

steatopygous - having fat buttocks

So now you know, and you know what they say about knowing...