This is a word — meaning to have well-shaped buttocks — about which it would be possible to generate many puns, thereby making an ass of oneself and becoming the butt of jokes. The subject matter, and the rather beautiful form of the word itself, has lent itself to adoption by word-hungry authors with erotic intent.
Early references were specifically to the callipygian Venus, that famous Roman statue often called the Venus Kallipygos in which the anonymous lady, possibly a mortal woman and not a deity, is staring back over her shoulder. It was only in the 1880s that it began to be applied more widely:
The article purveyed was a piece of highly starched buckram, attached by strings around the waist at the back of the underskirt, to give Callipygian comeliness to the form — in brief, Marie Antoinette's fichu polisson was what in the present French nomenclature is termed a tournure.
The Fortnightly, Aug. 1887. A tournure is a bustle.
This is a more modern example:
Those dusky Afro-Scandinavian buttocks, which combine the callipygian rondure observed among the races of the Dark Continent with the taut and noble musculature of sturdy Olaf, our blond Northern cousin.
Gravity’s Rainbow, by Thomas Pynchon, 1973.
The origin of the word is in Greek kallipugos, as in the statue; that derives from kallos, beauty (as in calligraphy, or callisthenics, or the lily called hemerocallis) plus puge, buttocks.