Princeton's WordNet(4.00 / 2 votes)Rate this definition: 1  unfathomable(adj) of depth; not capable of being sounded or measured. 2  abysmal, abyssal, unfathomable(adj) resembling an abyss in depth; so deep as to be unmeasurable. 3  unfathomable(adj) impossible to come to understand.
The rectus abdominis, transverse abdominis, external oblique and the internal oblique make up the abdominal muscles. The internal obliques are located just underneath the external obliques and allow you to bend from the side, as well as twist your torso.
External Oblique Muscle: Functional Anatomy Guide. The external oblique (L. externus, external ; obliquus, oblique.) is a broad, flat sheet of muscle on either side of the abs. It is named after its location (external to the internal oblique) and its fiber direction (oblique relative to the midline of the body).