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.
The external oblique muscle is one of the largest parts of the trunk area. Each side of the body has an external oblique muscle. The external oblique muscle is one of the outermost abdominal muscles, extending from the lower half of the ribs around and down to the pelvis.