a diagonally arranged abdominal muscle on either side of the torso

The external oblique muscle (of the abdomen) (also external abdominal oblique muscle) is the largest and the most superficial of the three flat muscles of the lateral anterior abdomen. The American Heritage Stedman's Medical Dictionary defines abdominal external oblique muscle as "A muscle with origin from the fifth to twelfth ribs, with insertion into the anterior lateral lip of the iliac crest, the inguinal ligament, and the anterior layer of the sheath of the rectus muscle of the abdomen, with nerve supply from the ventral branches of the lower thoracic nerves, and whose action diminishes the capacity of the abdomen and draws the chest down."