1. of depth; not capable of being sounded or measured • Ant: fathomable • Similar to: unsoundable • See Also: deep 2. resembling an abyss in depth; so deep as to be unmeasurable - the abyssal depths of the ocean • Syn: abysmal, abyssal • Similar to: deep • Derivationally related forms: abyss (for: abyssal), abysm (for: abysmal) 3. impossible to come to understand • Similar to: incomprehensible, uncomprehensible