External oblique (note 1)

part of your "abs"; partly responsible for twisting and bending the torso at the waist. Abdominals are important in archery, particularly on the bow side, as they help stabilise the torso and keep the body upright. It's bad form to lean away from the bow (mechanically inefficient and causes the string to contact the body) and this, with the rectus abdominis and some of the back muscles, helps avoid that.

Note 1: Sometimes "obliquus abdominis"