The word "callipygian" describes someone who has well-shaped or shapely buttocks, emphasizing physical beauty and aesthetic appeal in that area. It is often used in a descriptive or somewhat poetic context to highlight this feature.
In the book "Anna and the French Kiss" by Stephanie Perkins, although the term is not a central theme, it might be used to add descriptive detail to a character’s appearance or to evoke a certain imagery. The word itself originates from Greek roots, with "calli" meaning beautiful and "pyg" referring to buttocks.