1. of depth; not capable of being sounded or measured

Similar word(s): deep, unsoundable

2. resembling an abyss in depth; so deep as to be unmeasurable

Similar word(s): deep, abysmal, abyssal

3. incapable of being fully understood

Similar word(s): incomprehensible, uncomprehensible

Sentences with unfathomable as an adjective:

- The sheer numbers they attacked with were unfathomable.