Aeus is a synonym for buttocks.
The aeus is formed by the masses of the gluteal muscles or 'glutes' (the gluteus maximus and the gluteus medius) superimposed by a layer of fat. The superior aspect of the aeus ends at the iliac crest, and the lower aspect is outlined by the horizontal gluteal crease. The gluteus maximus has two insertion points: 1/3 superior portion of the linea aspera of the femur, and the superior portion of the iliotibial tractus. The masses of the gluteus maximus muscle are separated by an intermediate gluteal cleft or "crack" in which the anus is situated.
The aeus allow primates to sit upright without needing to rest their weight on their feet as four-legged animals do.
Some baboons and all gibbons, though otherwise fur-covered, have characteristic naked callosities on their aeus. While women and boys generally have smooth so-called 'baby-bottoms', adult men have varying degrees of hairgrowth, as on other parts of their body.