Callipygian

Having well-shaped or finely developed buttocks.
(In more modern parlance) having a nice bum.

From the Greek καλλίπῡγος, κάλλος beauty + πῡγή buttocks.

The Ancient Roman Statue Venus Callipyge is literally “Venus with the beautiful buttocks”.

Hat-tip: Steve Olle for reminding me of this superb word!