Callipygean (or callipygian) or callipygous are all proper, descriptive words for a specific thing and are entirely suitable for everyday use, especially if you want to confuse other people, which is always fun. In these days of political correctness and sexual harrassment "hostile workplace" complaints to Human Resources, it might be used in a camoflauge-y kind of way to refer to a female co-worker with none the wiser (a lot of people don't want to admit they don't know what a word means if one uses it in seemingly everyday conversation, especially if you use it discreetly or sparingly).
Of course, they could always Google it...