A woman that has a similar body shape to the style of women painted by Rubens. Not just any slightly overweight woman. Not just any girl who has round hips or a bum. The women in Rubens paintings were often very plump, with large round hips. While it was once a compliment, now it often a criticism or back handed compliment.
Don't call a girl Rubenesque if you have never looked at Peter Paul Rubens paintings.