Mac Dictionary’s definition:
having well-shaped buttocks.
callipygous |-ˈpīgəs| adjective
ORIGIN late 18th cent.: from Greek kallipūgos (used to describe a famous statue of Venus), from kallos ‘beauty’ + pūgē ‘buttocks,’ + -ian.
Well, gentlemen, I think we’ve found a politer way to tell a girl she has a nice ass:
“Dude, you know you want her, she’s extremely callipygian.”