from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Difficult or impossible to understand; incomprehensible: unfathomable theories.
- adj. Difficult or impossible to measure: the unfathomable depths.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. impossible to fathom or understand; incomprehensible
- adj. difficult to penetrate.
from The Century Dictionary and Cyclopedia
- Incapable of being fathomed or sounded; too deep to be measured; hence, not capable of being sounded by thought or comprehended.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- adj. of depth; not capable of being sounded or measured
- adj. resembling an abyss in depth; so deep as to be unmeasurable
- adj. impossible to come to understand
un- + fathomable (Wiktionary)