Usage of Callipygian / Callipygous

Callipygian and callipygous have virtually the same definition and are interchangeable.

callipygian, adj.

Designating a person who has well-shaped or finely developed buttocks; of or relating to such a person. Also: of or relating to the buttocks.

callipygous, adj.
_{Compare earlier CALLIPYGIAN adj.}

Designating a person who has well-shaped or finely developed buttocks; of or relating to such a person; = CALLIPYGIAN adj.

_{Source: Oxford English Dictionary (login required)}

Here’s an example usage offered by the OED:

_{2011   New Yorker 28 Mar. 60/3   The famously callipygian television personality Kim Kardashian makes frequent requests for free Spanx.}

You can swap in — for the sake of adjectival illustration — fine-butted wherever you see either to see that the usage is correct. For example:

The famously fine-butted television personality Kim Kardashian makes frequent requests for free Spanx.

Why do the cameramen at these sports events seem so adroit at focusing on the most fine-butted of the ladies?

To know before you go . . .

Google Books Ngram Viewer indicates that callipygian is more common.

If you need an adverb, the OED lists the derivative adverb callipygously. Callipygianously is not in use.

Callipygous is pronounced cal-uh-PIE-gus (ˌkæləˈpaɪɡəs), with a hard g and a long-i y. Callipygian has a soft g and a short-i y (ˌkæləˈpɪdʒiən). (Hear the pronunciations.)