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.
Abdominal external oblique muscle. The external oblique muscle (of the abdomen) (also external abdominal oblique muscle) is the largest and the most superficial (outermost) of the three flat muscles of the lateral anterior abdomen.